[APM] Add known issue for HTTP/2 strict client framing failures

[carsonip]

Summary

• Add an APM known-issue entry for HTTP/2 framing/protocol failures seen with strict clients.
• Document affected 8.x versions, user-visible symptoms, workaround guidance, and the 8.19.15 fix version.
• Exclude 9.x version mentions to keep this scoped to the 8.x docs stream.

Corresponds to 9.x doc change elastic/apm-server#20911

Part of elastic/apm-server#20887

Made with Cursor

[github-actions]

🤖 GitHub comments

Just comment with:

• run docs-build : Re-trigger the docs validation. (use unformatted text in the comment!)

[vishaangelova]

Left a couple of formatting suggestions, but otherwise LGTM.

[carsonip]

@Mergifyio backport 8.19

[mergify]

backport 8.19

✅ Backports have been created

Details

• #4980 [8.19] (backport #4979) [APM] Add known issue for HTTP/2 strict client framing failures has been created for branch 8.19 but encountered conflicts

Cherry-pick of 81a1950 has failed:

On branch mergify/bp/8.19/pr-4979
Your branch is up to date with 'origin/8.19'.

You are currently cherry-picking commit 81a19502.
  (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)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   docs/en/observability/apm/known-issues.asciidoc

no changes added to commit (use "git add" and/or "git commit -a")

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