RFR: 8321937: GenShen: Sync up 21u based repo [v2]

[William Kemper]

> Backport recent Shenandoah changes

William Kemper has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains 14 additional commits since the last revision:

 - Merge remote-tracking branch 'shenandoah-jdk21u/master' into merge-shen-master-into-shen-21
 - Use API for version of jtreg used to build jdk21
 - 8316226: GenShen: Consider forcing auto-tenure age to be greater than 1
   
   Reviewed-by: wkemper, kdnilsen
 - 8320336: GenShen: Reduce proactive humongous defragmentation efforts
   
   Reviewed-by: wkemper, ysr
 - 8320112: GenShen: Improve end of process report
   
   Reviewed-by: kdnilsen
 - 8320119: GenShen: Correct misspellings of parsable
   
   Reviewed-by: kdnilsen, ysr
 - 8319342: GenShen: Reset the count of degenerated cycles in a row following Full GC
   
   Reviewed-by: wkemper, ysr
 - 8319931: GenShen: Increase no progress threshold for TestThreadFailure
   
   Reviewed-by: kdnilsen
 - 8319867: GenShen: Make old regions parsable at end of concurrent cycles
   
   Reviewed-by: kdnilsen, ysr
 - 8318462: [GenShen] Prevent unsafe access to displaced mark-word
   
   Reviewed-by: kdnilsen, wkemper, ysr
 - ... and 4 more: https://git.openjdk.org/shenandoah-jdk21u/compare/9b613688...555c581a

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

Changes:
  - all: https://git.openjdk.org/shenandoah-jdk21u/pull/4/files
  - new: https://git.openjdk.org/shenandoah-jdk21u/pull/4/files/0afff542..555c581a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=4&range=01
 - incr: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=4&range=00-01

  Stats: 17927 lines in 348 files changed: 11041 ins; 1810 del; 5076 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/4.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/4/head:pull/4

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/4