RFR: merge openjdk/jdk:master

[William Kemper]

Merge jdk+21-0. There is a small change in the generational mode to use a different API for iterating java threads to resolve a merge conflict.

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Commit messages:
 - Replace use of removed API
 - Merge tag 'jdk-21+0' into merge-jdk21-0
 - 8297642: PhaseIdealLoop::only_has_infinite_loops must detect all loops that never lead to termination
 - 8298255: JFR provide information about dynamization of number of compiler threads
 - 8298383: JFR: GenerateJfrFiles.java lacks copyright header
 - 8298379: JFR: Some UNTIMED events only sets endTime
 - 8298129: Let checkpoint event sizes grow beyond u4 limit
 - 8297718: Make NMT free:ing protocol more granular
 - 8298173: GarbageCollectionNotificationContentTest test failed: no decrease in Eden usage
 - 8298272: Clean up ProblemList
 - ... and 163 more: https://git.openjdk.org/shenandoah/compare/35b26d60...63a32877

Changes: https://git.openjdk.org/shenandoah/pull/183/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=183&range=00
  Stats: 81449 lines in 1217 files changed: 38340 ins; 35581 del; 7528 mod
  Patch: https://git.openjdk.org/shenandoah/pull/183.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah pull/183/head:pull/183

PR: https://git.openjdk.org/shenandoah/pull/183