RFR: Merge openjdk/jdk21u-dev:master

[William Kemper]

Merges tag jdk-21.0.6+2

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Commit messages:
 - 8342681: TestLoadBypassesNullCheck.java fails improperly specified VM option
 - 8341235: Improve default instruction frame title in PassFailJFrame
 - 8340799: Add border inside instruction frame in PassFailJFrame
 - 8340812: LambdaForm customization via MethodHandle::updateForm is not thread safe
 - 8339803: Acknowledge case insensitive unambiguous keywords in tzdata files
 - 8306446: java/lang/management/ThreadMXBean/Locks.java transient failures
 - 8333728: ubsan: shenandoahFreeSet.cpp:1347:24: runtime error: division by zero
 - 8342496: C2/Shenandoah: SEGV in compiled code when running jcstress
 - 8340214: C2 compilation asserts with "no node with a side effect" in PhaseIdealLoop::try_sink_out_of_loop
 - 8337066: Repeated call of StringBuffer.reverse with double byte string returns wrong result
 - ... and 27 more: https://git.openjdk.org/shenandoah-jdk21u/compare/7dc0f7a6...bb60a427

The webrev contains the conflicts with master:
 - merge conflicts: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=136&range=00.conflicts

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/136/files
  Stats: 12428 lines in 75 files changed: 11328 ins; 720 del; 380 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/136.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/136/head:pull/136

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/136