[8.19] (backport #4979) [APM] Add known issue for HTTP/2 strict client framing failures

[mergify]

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

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This is an automatic backport of pull request #4979 done by Mergify.

[mergify]

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	both modified:   docs/en/observability/apm/known-issues.asciidoc

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