[8.14] Add allow_older_versions setting (backport #1205)

[mergify]

This updates the Fleet & Agent docs with the allow_older_version setting that was added via elastic/beats#36884

Closes: #1135
Related Beats docs PR: elastic/beats#40338

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Fleet settings -> Elasticsearch output settings -> Advanced YAML configuration

Configure standalone Elastic Agent -> Outputs -> Elasticsearch

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This is an automatic backport of pull request #1205 done by Mergify.

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