RFR: 8334491: GenShen: Revert changes to Shenandoah defaults

[William Kemper]

Clean backport.

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Commit messages:
 - Backport 744fc265aafe2fd59c2ca53c798ee930119f5d8d

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/66/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=66&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8334491
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/66.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/66/head:pull/66

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/66