RFR: Merge openjdk/jdk21u-dev:master

[William Kemper]

Merges tag jdk-21.0.6+4

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Commit messages:
 - 8343923: GHA: Switch to Xcode 15 on MacOS AArch64 runners
 - 8338389: [JFR] Long strings should be added to the string pool
 - 8342823: Ubsan: ciEnv.cpp:1614:65: runtime error: member call on null pointer of type 'struct CompileTask'
 - 8338449: ubsan: division by zero in sharedRuntimeTrans.cpp
 - 8336911: ZGC: Division by zero in heuristics after JDK-8332717
 - 8344164: [s390x] ProblemList hotspot/jtreg/runtime/NMT/VirtualAllocCommitMerge.java
 - 8343724: [PPC64] Disallow OptoScheduling
 - 8342607: Enhance register printing on x86_64 platforms
 - 8343506: [s390x] multiple test failures with ubsan
 - 8342183: Update tests to use stronger algorithms and keys
 - ... and 10 more: https://git.openjdk.org/shenandoah-jdk21u/compare/9bdf2a61...4b1365ea

The merge commit only contains trivial merges, so no merge-specific webrevs have been generated.

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/138/files
  Stats: 3648 lines in 121 files changed: 1674 ins; 439 del; 1535 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/138.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/138/head:pull/138

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/138