RFR: 8377043: Shenandoah: Convert ShenandoahHeapRegion related code to use Atomic<T>

[Ben Taylor]

Modernizes ShenandoahHeapRegion code by replacing `AtomicAccess` methods with `Atomic<T>` types.

Tier1 test results on x86_64 linux are the same before and after this change when running with `TEST_VM_OPTS=-XX:+UseShenandoahGC`.

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Commit messages:
 - 8377043: Convert ShenandoahHeapRegion related code to use Atomic<T>

Changes: https://git.openjdk.org/jdk/pull/29575/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29575&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8377043
  Stats: 37 lines in 4 files changed: 2 ins; 2 del; 33 mod
  Patch: https://git.openjdk.org/jdk/pull/29575.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29575/head:pull/29575

PR: https://git.openjdk.org/jdk/pull/29575