RFR: 8325081: Move '_soft_ref_policy' to 'CollectedHeap'

[Lei Zaakjyu]

trivial

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Commit messages:
 - move '_soft_ref_policy' to 'CollectedHeap'

Changes: https://git.openjdk.org/jdk/pull/17693/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17693&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8325081
  Stats: 49 lines in 13 files changed: 3 ins; 44 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/17693.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17693/head:pull/17693

PR: https://git.openjdk.org/jdk/pull/17693