[main](backport #41261) docs: Prepare Changelog for 8.15.3

[mergify]

Prepare Changelog for 8.15.3.

Merge once the BC that will be published is decided.

---

This is an automatic backport of pull request #41261 done by Mergify.

[mergify]

Cherry-pick of e29591e has failed:

On branch mergify/bp/main/pr-41261
Your branch is up to date with 'origin/main'.

You are currently cherry-picking commit e29591eeb3.
  (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:   CHANGELOG.asciidoc
	modified:   CHANGELOG.next.asciidoc

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   libbeat/docs/release.asciidoc

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

[cla-checker-service]

❌ Author of the following commits did not sign a Contributor Agreement:
c1e8928

Please, read and sign the above mentioned agreement if you want to contribute to this project

[botelastic]

This pull request doesn't have a Team:<team> label.

[mergify]

backport-8.x has been added to help with the transition to the new branch 8.x.
If you don't need it please use backport-skip label and remove the backport-8.x label.