[8.15] Add changelog for fallback to ILM if DSL not present (backport #13918)

[mergify]

Motivation/summary

APM-Server switched from Index Lifecycle Management(ILM) to Datastream Lifecycle (DSL) in v8.15.0. This switch was done when we moved from APM integration (which used ILM) to APM-data plugin (which uses DSL) in ES for managing APM datastreams. As a result of the switch, any old datastreams created before the switch would be Unmanaged because the datastream will never be updated with the DSL lifecycle -- this has to be done manually by using the PUT API.

Checklist

• Update CHANGELOG.asciidoc
• Documentation has been updated

How to test these changes

1. Create a stack (ES, Kibana, APM-Server) with data-persistence enabled for ES using 8.14.3 version. We use the 8.14.3 as that is the latest available version which uses APM integration package and thus configures ILM policies.

   Example docker-compose.yaml

   version: '3.9'
   x-logging: &default-logging
     driver: "json-file"
     options:
       max-size: "1g"
   services:
     elasticsearch:
       image: docker.elastic.co/elasticsearch/elasticsearch:8.14.3
       ports:
         - 9200:9200
       healthcheck:
         test: ["CMD-SHELL", "curl -s http://localhost:9200/_cluster/health?wait_for_status=yellow&timeout=500ms"]
         retries: 300
         interval: 1s
       environment:
         - "ES_JAVA_OPTS=-Xms1g -Xmx1g"
         - "network.host=0.0.0.0"
         - "transport.host=127.0.0.1"
         - "http.host=0.0.0.0"
         - "cluster.routing.allocation.disk.threshold_enabled=false"
         - "discovery.type=single-node"
         - "xpack.security.authc.anonymous.roles=remote_monitoring_collector"
         - "xpack.security.authc.realms.file.file1.order=0"
         - "xpack.security.authc.realms.native.native1.order=1"
         - "xpack.security.enabled=true"
         - "xpack.license.self_generated.type=trial"
         - "xpack.security.authc.token.enabled=true"
         - "xpack.security.authc.api_key.enabled=true"
         - "logger.org.elasticsearch=${ES_LOG_LEVEL:-error}"
         - "action.destructive_requires_name=false"
       volumes:
         - "./testing/docker/elasticsearch/roles.yml:/usr/share/elasticsearch/config/roles.yml"
         - "./testing/docker/elasticsearch/users:/usr/share/elasticsearch/config/users"
         - "./testing/docker/elasticsearch/users_roles:/usr/share/elasticsearch/config/users_roles"
         - "./testing/docker/elasticsearch/ingest-geoip:/usr/share/elasticsearch/config/ingest-geoip"
         - "/Users/lahsivjar/Projects/elastic/tmp/esdata2:/usr/share/elasticsearch/data"
       logging: *default-logging
   
     kibana:
       image: docker.elastic.co/kibana/kibana:8.14.3
       ports:
         - 5601:5601
       healthcheck:
         test: ["CMD-SHELL", "curl -s http://localhost:5601/api/status | grep -q 'All services are available'"]
         retries: 300
         interval: 1s
       environment:
         ELASTICSEARCH_HOSTS: '["http://elasticsearch:9200"]'
         ELASTICSEARCH_USERNAME: "${KIBANA_ES_USER:-kibana_system_user}"
         ELASTICSEARCH_PASSWORD: "${KIBANA_ES_PASS:-changeme}"
         XPACK_FLEET_AGENTS_ELASTICSEARCH_HOSTS: '["http://elasticsearch:9200"]'
       depends_on:
         elasticsearch: { condition: service_healthy }
       volumes:
         - "./testing/docker/kibana/kibana.yml:/usr/share/kibana/config/kibana.yml"
       logging: *default-logging
   
     apm-server:
       image: docker.elastic.co/apm/apm-server:8.14.3
       ports:
         - 8200:8200
       healthcheck:
         test: ["CMD-SHELL", "bash -c 'echo -n > /dev/tcp/127.0.0.1/8200'"]
         retries: 300
         interval: 1s
       depends_on:
         elasticsearch: { condition: service_healthy }
       volumes:
         - "./testing/docker/apm-server/apm-server.yml:/usr/share/apm-server/apm-server.yml"
       logging: *default-logging

   NOTE: The config files used in the example docker-compose are [available here](https://github.com/elastic/apm-server/tree/main/testing/docker). `apm.server.yml` file used in the docker-compose could be a simple config file:

   apm-server:
     host: "0.0.0.0:8200"
   output.elasticsearch:
     hosts: ["elasticsearch:9200"]
     username: "admin"
     password: "changeme"
   logging.level: info
   logging.to_stderr: true

2. Install the APM integration in the cluster.

3. Send some data, for example: by using apmsoak. Example command: go run ./cmd/apmsoak/ run --file cmd/apmsoak/scenarios.yml --scenario apm-server --server-url http://localhost:8200

4. Assert that the APM indices created are managed by ILM, for example: by running GET /_data_stream/traces-apm-default to check for trace indices

5. Build an Elasticsearch docker image using the branch in this PR: ./gradlew buildAarch64DockerImage

6. Update the versions used in the stack created in step 1 to 8.16.0-SNAPSHOT, for ES use the docker image built in step 5

7. Send some more data as we did in step 3

8. Assert that all the APM indices are still managed by ILM

9. Rollover the datastream

10. Assert that all the APM indices, including the one created using rollover in step 9, are still managed by ILM

Also, test if the setup works by itself i.e. if a cluster is created using the latest version (with the changes in the PR) then it works as expected and the created APM indices in this case are managed by DSL (datastream lifecycle).

NOTE: Any indices created when APM is on version 8.15.0 and datastream created before 8.15.0 i.e. with ILM, will remain Unmanaged even after this fix. To fix them, we would need to explicitly update them OR use the PUT API on datastream to set DSL.

Related issues

• New indexes created for datastreams after update to 8.15.0 are without lifecycle policies #13898
• [plugin/apm-data] Lifecycle for APM-Server datastreams switches to Unmanaged on upgrade elasticsearch#112137

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This is an automatic backport of pull request #13918 done by [Mergify](https://mergify.com).