RFR: 8368501: Shenandoah: GC progress evaluation does not use generation

[William Kemper]

Remove unused `generation` parameter from stop-the-world GC progress evaluation. Also, have `shMetrics` use the freeset for `usage`  details. This is a follow up to a comment from an unrelated PR: https://github.com/openjdk/jdk/pull/27456#issuecomment-3325724362.

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Commit messages:
 - Have stw progress evaluation just use the free set
 - Use collected generation to assess "used" progress

Changes: https://git.openjdk.org/jdk/pull/27561/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27561&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8368501
  Stats: 79 lines in 4 files changed: 6 ins; 37 del; 36 mod
  Patch: https://git.openjdk.org/jdk/pull/27561.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27561/head:pull/27561

PR: https://git.openjdk.org/jdk/pull/27561