RFR: 8321605: GenShen: Old generation reference process is never reset

[William Kemper]

8321605: GenShen: Old generation reference process is never reset

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Commit messages:
 - Backport 2458f13684d9c2665c7c86dffcc23a24d1e73797

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/16/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=16&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8321605
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/16.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/16/head:pull/16

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/16