RFR: 8358529: GenShen: Heuristics do not respond to changes in SoftMaxHeapSize

[William Kemper]

Clean backport. Makes generational mode honor `SoftMaxHeapSize`.

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Commit messages:
 - Backport fa32bfe11300fdadb35f083037f6ab2a8985d210

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/202/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=202&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8358529
  Stats: 177 lines in 17 files changed: 78 ins; 32 del; 67 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/202.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/202/head:pull/202

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/202