RFR: 8325670: GenShen: Allow old to expand at end of each GC

[William Kemper]

Clean backport

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Commit messages:
 - Backport 2f1fa6dbe2b6dcea15603ac42087af83961622f9

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/24/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=24&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8325670
  Stats: 8 lines in 1 file changed: 5 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/24.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/24/head:pull/24

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/24