RFR: Merge openjdk/jdk:master [v2]

[William Kemper]

> Merges tag jdk-21+21

William Kemper has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 278 commits:

 - Merge tag 'jdk-21+21' into merge-jdk-21-21
   
   Added tag jdk-21+21 for changeset 705ad7d8
 - Alphabetize includes
   
   Reviewed-by: shade
 - Use distinct "end of cycle" message for each Shenandoah pause
   
   Reviewed-by: kdnilsen, ysr
 - Remove the remaining unclean upstream difference
   
   Reviewed-by: kdnilsen
 - Merge openjdk/jdk:master
   
   Reviewed-by: shade
 - Add generations to freeset
   
   Reviewed-by: ysr, wkemper
 - Cleanup NumberSeq additions
   
   Reviewed-by: ysr, rkennke
 - Remove redundant initialization code
   
   Reviewed-by: shade
 - Usage tracking cleanup
   
   Reviewed-by: kdnilsen, ysr
 - Use state of mark context itself to track stability of old mark bitmap
   
   Reviewed-by: ysr, shade
 - ... and 268 more: https://git.openjdk.org/shenandoah/compare/705ad7d8...b701c556

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Changes: https://git.openjdk.org/shenandoah/pull/276/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=276&range=01
  Stats: 19788 lines in 216 files changed: 17933 ins; 888 del; 967 mod
  Patch: https://git.openjdk.org/shenandoah/pull/276.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/276/head:pull/276

PR: https://git.openjdk.org/shenandoah/pull/276