RFR: 8329342: GenShen: Synchronize shenandoah-jdk21u:master with shenandoah:master [v2]

[William Kemper]

> Pull in recent changes from shenandoah:master, _en masse_.

William Kemper has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 28 commits:

 - Sync up gtest test
 - Merge remote-tracking branch 'shenandoah-21/master' into synch-up-with-shenandoah-master
 - Merge
 - Merge
 - Fix up tests
 - Sync up with shenandoah tip
 - Merge remote-tracking branch 'openjdk-21u-dev/master' into synch-up-with-shenandoah-master
 - 8324325: [Genshen] Normalize wrt AgeTable changes from JDK-8314329
   
   Make GenShen consistent with changes in JDK-8314329.
   
   Reviewed-by: shade
 - 8325670: GenShen: Allow old to expand at end of each GC
   
   Backport-of: 2f1fa6dbe2b6dcea15603ac42087af83961622f9
 - 8326626: GenShen: Remove dead code associated with non-elastic TLABS
   
   Backport-of: 9fde64ecf7f4954dbd4028a57cdeb400de8b0268
 - ... and 18 more: https://git.openjdk.org/shenandoah-jdk21u/compare/f2a013bd...6e465522

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Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/29/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=29&range=01
  Stats: 23634 lines in 234 files changed: 21102 ins; 1378 del; 1154 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/29.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/29/head:pull/29

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/29