RFR: 8337511: Implement JEP-404: Generational Shenandoah (Experimental)

[William Kemper]

This PR merges JEP 404, a generational mode for the Shenandoah garbage collector. The JEP can be viewed here: https://openjdk.org/jeps/404. We would like to target JDK24 with this PR.

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Commit messages:
 - 8337511: Implement JEP-404: Generational Shenandoah (Experimental)

Changes: https://git.openjdk.org/jdk/pull/21273/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21273&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8337511
  Stats: 22576 lines in 229 files changed: 20937 ins; 807 del; 832 mod
  Patch: https://git.openjdk.org/jdk/pull/21273.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21273/head:pull/21273

PR: https://git.openjdk.org/jdk/pull/21273