[8.14] fix: Update FlushBytes parsing/defaults (backport #13576)

[mergify]

Motivation/summary

Updates the FlushBytes setting to default to 1 mib and only override to 24kb if the user has explicitly set it to 24kb.

Checklist

• Update CHANGELOG.asciidoc
  - [ ] Documentation has been updated

How to test these changes

1. Spin up an APM Server and send data to it (without this fix).
2. Look at the request length for Elasticsearch requests and verify they're smaller than 50kb and very close to 24kb (26kb)
3. Now start apm sever with the code in this PR and send data to it
4. Verify that the flush size is significantly higher than before and if you sent enough data it should be ~1MB.

Related issues

Fixes #13024

---

This is an automatic backport of pull request #13576 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of a453a88 has failed:

On branch mergify/bp/8.14/pr-13576
Your branch is up to date with 'origin/8.14'.

You are currently cherry-picking commit a453a8850.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   internal/beater/beater_test.go

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   changelogs/head.asciidoc
	both modified:   internal/beater/beater.go

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[carsonip]

is 8.14 the right version?

edit: ack 8.14.3

[inge4pres]

is 8.14 the right version?

I think so, because this is a backport, and we don't release 8.13 anymore?

Releasing an 8.14.3. So this needs to be merged into 8.14

[marclop]

Tested this on ESS with a 1GB APM Server and an 8GB Elasticsearch. Documents per bulk request is up to 4-5K and median request length is 800-1MB, so the small bulk_indexer size regression is gone.