RFR: 8364934: G1: Rename members of G1CollectionSet

[Thomas Schatzl]

Hi all,

  please review this renaming of various members of `G1CollectionSet` to

- use the same prefixes for the regions ("_regions") and candidate groups ("_groups") consistently, avoiding adding prefixes here and there for no discernible reason
- remove unnecessary prefixes (`collection_set`) - everything is part of the collection set
- fix/improve some documentation

Testing: gha

Thanks,
  THoams

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Depends on: https://git.openjdk.org/jdk/pull/26659

Commit messages:
 - 8364934

Changes: https://git.openjdk.org/jdk/pull/26670/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26670&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8364934
  Stats: 106 lines in 6 files changed: 15 ins; 4 del; 87 mod
  Patch: https://git.openjdk.org/jdk/pull/26670.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26670/head:pull/26670

PR: https://git.openjdk.org/jdk/pull/26670