RFR: 8375978: G1: Convert G1Policy to use Atomic<T>

[Thomas Schatzl]

Hi all,

  please review this conversion of `G1Policy` to use `Atomic<T>`.

Also removed the unused `_young_list_max_length` instead of unnecessarily converting it.

Testing: gha

Thanks,
  Thomas

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Commit messages:
 - 8375978

Changes: https://git.openjdk.org/jdk/pull/29344/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29344&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8375978
  Stats: 10 lines in 2 files changed: 0 ins; 3 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/29344.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29344/head:pull/29344

PR: https://git.openjdk.org/jdk/pull/29344