RFR: 8337511: Implement JEP 404: Generational Shenandoah (Experimental) [v6]

[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.

William Kemper has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 503 commits:

 - Merge openjdk/jdk tip into great-genshen-pr-redux
 - Merge remote-tracking branch 'jdk/master' into merge-latest
 - Merge remote-tracking branch 'jdk/master' into merge-latest
 - Merge
 - 8342861: GenShen: Old generation in unexpected state when abandoning mixed gc candidates
   
   Reviewed-by: kdnilsen
 - 8342734: GenShen: Test failure gc/shenandoah/TestReferenceRefersToShenandoah.java#generational
   
   Reviewed-by: ysr
 - 8342919: GenShen: Fix whitespace
   
   Reviewed-by: xpeng, kdnilsen
 - 8342927: GenShen: Guarantee slices of time for coalesce and filling
   
   Reviewed-by: kdnilsen
 - 8342924: GenShen: Problem list gc/shenandoah/TestReferenceRefersToShenandoah.java
   
   Reviewed-by: kdnilsen, ysr
 - 8342848: Shenandoah: Marking bitmap may not be completely cleared in generational mode
   
   Reviewed-by: wkemper
 - ... and 493 more: https://git.openjdk.org/jdk/compare/1c448347...19b25bc3

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Changes: https://git.openjdk.org/jdk/pull/21273/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21273&range=05
  Stats: 22859 lines in 229 files changed: 21116 ins; 886 del; 857 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