RFR: 8338620: GenShen: Distinguish between young and old collector in GC configuration

[William Kemper]

Clean backport.

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Commit messages:
 - Backport e31d1e49885f02ae25bd5758c8d4a2705f4b97e1

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/91/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=91&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338620
  Stats: 38 lines in 2 files changed: 24 ins; 14 del; 0 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/91.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/91/head:pull/91

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/91