RFR: 8373087: Parallel: Rename PSGenerationPool to PSOldGenerationPool [v2]

[Albert Mingkun Yang]

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

Albert Mingkun Yang has updated the pull request incrementally with one additional commit since the last revision:

  review

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Changes:
  - all: https://git.openjdk.org/jdk/pull/28657/files
  - new: https://git.openjdk.org/jdk/pull/28657/files/c7922241..0393cec6

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28657&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28657&range=00-01

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 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