Changelog: Add 8.18.8 release notes

[inge4pres]

Motivation/summary

Add release notes for 8.18.8 release

Checklist

• Update CHANGELOG.asciidoc
• Documentation has been updated

For functional changes, consider:

• Is it observable through the addition of either logging or metrics?
• Is its use being published in telemetry to enable product improvement?
• Have system tests been added to avoid regression?

How to test these changes

Related issues

[mergify]

This pull request is now in conflicts. Could you fix it @inge4pres? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b changelog-8.18.8 upstream/changelog-8.18.8
git merge upstream/8.18
git push upstream changelog-8.18.8

[inge4pres]

retrying the CI jobs, apparently broken by ES not by us

[carsonip]

Suggested change

	- APM Integration on ECH or ECE uses recommended frozen values for infrastructure related configs. See the https://github.com/elastic/kibana/issues/234384[Kibana PR #234384].
	- APM Integration on ECH or ECE uses recommended frozen values for infrastructure related configs. The fix is applied through https://github.com/elastic/kibana/pull/235306[kibana#235306]