[jdk21u-dev] RFR: 8314629: Generational ZGC: Clearing All SoftReferences log line lacks GCId

[Liang Mao]

Clean backport of enhancement of generational ZGC to fix the GC ID of SoftReferences clearing.

Additional testing:
 - [x] Linux aarch64 server fastdebug, `hotspot_gc` with `-XX:+UseZGC +XX+ZGenerational`

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Commit messages:
 - Backport 88dafe564f163ed738a8cb6adc449b94e606999f

Changes: https://git.openjdk.org/jdk21u-dev/pull/271/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=271&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8314629
  Stats: 9 lines in 2 files changed: 8 ins; 1 del; 0 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/271.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/271/head:pull/271

PR: https://git.openjdk.org/jdk21u-dev/pull/271