RFR: 8321937: GenShen: Sync up 21u based repo

[William Kemper]

Backport recent Shenandoah changes

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Commit messages:
 - Use API for version of jtreg used to build jdk21
 - 8316226: GenShen: Consider forcing auto-tenure age to be greater than 1
 - 8320336: GenShen: Reduce proactive humongous defragmentation efforts
 - 8320112: GenShen: Improve end of process report
 - 8320119: GenShen: Correct misspellings of parsable
 - 8319342: GenShen: Reset the count of degenerated cycles in a row following Full GC
 - 8319931: GenShen: Increase no progress threshold for TestThreadFailure
 - 8319867: GenShen: Make old regions parsable at end of concurrent cycles
 - 8318462: [GenShen] Prevent unsafe access to displaced mark-word
 - 8319671: GenShen: Old marking may encounter invalid pointers in SATB buffers
 - ... and 3 more: https://git.openjdk.org/shenandoah-jdk21u/compare/57fb4b2b...0afff542

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/4/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=4&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8321937
  Stats: 925 lines in 38 files changed: 438 ins; 225 del; 262 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