RFR: 8337511: Implement JEP 404: Generational Shenandoah (Experimental) [v8]

[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 524 commits:

 - Merge remote-tracking branch 'shenandoah/master' into great-genshen-pr-redux
 - 8344670: GenShen: Use concurrent worker session for concurrent mark phase
   
   Reviewed-by: kdnilsen
 - 8344640: GenShen: Reuse existing card mark barrier function when dropping references
   
   Reviewed-by: shade, ysr
 - 8344592: GenShen: Remove unnecessary comments and changes
   
   Reviewed-by: kdnilsen
 - 8344263: GenShen: Reduce extraneous log messages at INFO level
   
   Reviewed-by: ysr, shade
 - 8344638: GenShen: Verifier should not touch claim token
   
   Reviewed-by: shade
 - Merge
 - 8344320: GenShen: Possible null pointer usage in shGenerationalHeap
   
   Reviewed-by: wkemper, ysr
 - 8344321: GenShen: Fix various sonar scan warnings
   
   Reviewed-by: kdnilsen, shade
 - 8344264: GenShen: Improve comments and method names
   
   Reviewed-by: shade
 - ... and 514 more: https://git.openjdk.org/jdk/compare/95a00f8a...7ab16403

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Changes: https://git.openjdk.org/jdk/pull/21273/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21273&range=07
  Stats: 22960 lines in 230 files changed: 21196 ins; 892 del; 872 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