RFR: 8373087: Parallel: Rename PSGenerationPool to PSOldGenerationPool

[Albert Mingkun Yang]

Trivial renaming to make it explicit that this class is only for old-gen.

Test: GHA

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Commit messages:
 - pgc-rename

Changes: https://git.openjdk.org/jdk/pull/28657/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28657&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8373087
  Stats: 9 lines in 3 files changed: 0 ins; 0 del; 9 mod
  Patch: https://git.openjdk.org/jdk/pull/28657.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28657/head:pull/28657

PR: https://git.openjdk.org/jdk/pull/28657