RFR: 8367731: G1: Make G1CollectionSet manage the young gen cset group

[Thomas Schatzl]

Hi all,

  please review this change that lets the collection set manage the young cset candidate group directly. Currently everywhere the collection set (not "containing" the young cset candidate group) is used, we immediately do the same thing for the young gen candidate group in the same place.

This makes it kind of natural to put together (and the `G1CollectionSet` already manages adding new regions to the young gen cset group anyway, so that split in responsibility is questionable anyway).

Testing: gha

Thanks,
  Thomas

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Commit messages:
 - * comment update
 - * let collectionset manage the young regions group card set

Changes: https://git.openjdk.org/jdk/pull/27312/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27312&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8367731
  Stats: 67 lines in 9 files changed: 26 ins; 29 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/27312.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27312/head:pull/27312

PR: https://git.openjdk.org/jdk/pull/27312