docs: add APM HTTP/2 strict client known issue for 8.x

Document the HTTP/2 framing/protocol failure symptoms, workaround guidance, and 8.19.15 fix version in APM known issues without 9.x references.

Author: carsonip

docs/en/observability/apm/known-issues.asciidoc

@@ -21,6 +21,20 @@ _Versions: XX.XX.XX, YY.YY.YY, ZZ.ZZ.ZZ_
 // If applicable, link to fix
 ////
 
+[discrete]
+== HTTP/2 connections may fail with strict clients due to framing issues
+
+_Elastic Stack versions: 8.19.12, 8.19.13, 8.19.14_ +
+_Fixed in Elastic Stack version 8.19.15_
+
+The issue occurs when using APM Server over HTTP/2 with strict clients.
+Affected clients may report errors such as `Error in the HTTP2 framing layer`,
+`Failure when receiving data from the peer`, or browser `net::ERR_HTTP2_PROTOCOL_ERROR`.
+
+As a workaround, use HTTP/1.1 between clients and APM Server, or offload HTTP/2 through a non-strict load balancer or proxy.
+
+This issue is fixed in 8.19.15 (https://github.com/elastic/apm-server/pull/20908[elastic/apm-server#20908]).
+
 [discrete]
 == Tail Sampling may not compact / expired TTLs as quickly as desired, causing increased storage usage.