[jdk21u-dev] RFR: 8322957: Generational ZGC: Relocation selection must join the STS

[Aleksey Shipilev]

Clean backport to fix Generational ZGC. 

FYI @fisk, @stefank, I am picking this up to 21u. Tell me if this is a bad idea :)

Additional testing:
 - [x] Linux x86_64 server fastdebug, `hotspot_gc` with `+ZGenerational`
 - [ ] Linux x86_64 server fastdebug, `tier{1,2,3,4}`

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Commit messages:
 - Backport ba23025cd8a9c1af37afea6444ce5ea2ff41e5af

Changes: https://git.openjdk.org/jdk21u-dev/pull/189/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=189&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8322957
  Stats: 168 lines in 14 files changed: 127 ins; 20 del; 21 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/189.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/189/head:pull/189

PR: https://git.openjdk.org/jdk21u-dev/pull/189