RFR: 8375439: G1: Convert G1MonotonicArena class to use Atomic<T>

[Thomas Schatzl]

Hi all,

  please review the change to use `Atomic<T>` in `G1MonotonicArena`.

Testing: gha

Thanks,
  Thomas

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Commit messages:
 - * copyright update
 - 8375439

Changes: https://git.openjdk.org/jdk/pull/29277/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29277&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8375439
  Stats: 58 lines in 3 files changed: 1 ins; 2 del; 55 mod
  Patch: https://git.openjdk.org/jdk/pull/29277.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29277/head:pull/29277

PR: https://git.openjdk.org/jdk/pull/29277