RFR: 8321937: GenShen: Sync up 21u based repo [v3]

[William Kemper]

> Backport recent Shenandoah changes

William Kemper has updated the pull request incrementally with one additional commit since the last revision:

  Remove use of option which has not been backported yet

-------------

Changes:
  - all: https://git.openjdk.org/shenandoah-jdk21u/pull/4/files
  - new: https://git.openjdk.org/shenandoah-jdk21u/pull/4/files/555c581a..b6415bdd

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=4&range=02
 - incr: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=4&range=01-02

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/4.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/4/head:pull/4

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/4