[jdk21u-dev] RFR: 8317808: HTTP/2 stream cancelImpl may leave subscriber registered

[Alexey Bakhtin]

Clean backport of JDK-8317808

All jtreg tests for java.net.httpclient are passed

-------------

Depends on: https://git.openjdk.org/jdk21u-dev/pull/1417

Commit messages:
 - Backport 6273ab97dc1a0d3c1f51ba94694d9594dd7593d4

Changes: https://git.openjdk.org/jdk21u-dev/pull/1426/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=1426&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8317808
  Stats: 29 lines in 4 files changed: 22 ins; 4 del; 3 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/1426.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/1426/head:pull/1426

PR: https://git.openjdk.org/jdk21u-dev/pull/1426