RFR: 8321816: GenShen: Provide a minimum amount of time for an old collection to run

[William Kemper]

Resolved conflict with `can_unload_classes_normal`.

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Commit messages:
 - Backport 80f7780bfd2435486a5941629d6b4913b82e60b7

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/14/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=14&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8321816
  Stats: 83 lines in 6 files changed: 42 ins; 17 del; 24 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/14.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/14/head:pull/14

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/14