RFR: 8368599: ShenandoahConcurrentMark could use ThreadsClaimTokenScope

[Francesco Andreuzzi]

Replace `StrongRootsScope` with `ThreadsClaimTokenScope` in `ShenandoahConcurrentMark::finish_mark_work` to be more precise with what is actually needed and why. Similar to #27385.

Test: tier1, and tier[1,2,3]_gc_shenandoah

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Commit messages:
 - replace

Changes: https://git.openjdk.org/jdk/pull/27477/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27477&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8368599
  Stats: 2 lines in 1 file changed: 0 ins; 1 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/27477.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27477/head:pull/27477

PR: https://git.openjdk.org/jdk/pull/27477