RFR: 8329342: GenShen: Synchronize shenandoah-jdk21u:master with shenandoah:master

[William Kemper]

The delta for this fork has grown too great for individual backport PRs. This change synchronizes the `shenandoah-jdk21u` fork by essentially checking out the `shenandoah` directory from `shenandoah` and reverting the few things we can't take because of changes outside of `shenandoah`.

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Commit messages:
 - 8324325: [Genshen] Normalize wrt AgeTable changes from JDK-8314329
 - Sync up gtest test
 - Fix up tests
 - Sync up with shenandoah tip

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/31/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=31&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8329342
  Stats: 5656 lines in 76 files changed: 2945 ins; 2220 del; 491 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/31.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/31/head:pull/31

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/31