Integrated: 8332081: GenShen: Disentangle generational mode update references

[William Kemper]

No conflicts after rebasing on recent PRs.

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Commit messages:
 - Backport def9d3feb212226688c6fd071723adc4e00cc361

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/49/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=49&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8332081
  Stats: 439 lines in 6 files changed: 237 ins; 185 del; 17 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/49.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/49/head:pull/49

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/49