RFR: 8369980: Use ThreadsClaimTokenScope in ShenandoahThreadRoots

[Francesco Andreuzzi]

The explicit usage of `Threads::change_thread_claim_token` and `Threads::assert_all_threads_claimed` in `ShenandoahThreadRoots` can be replaced with `ThreadsClaimTokenScope`.

Passes tier1, tier2, tier[1,2,3]_gc_shenandoah (fastdebug).

-------------

Commit messages:
 - replace

Changes: https://git.openjdk.org/jdk/pull/27845/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27845&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8369980
  Stats: 11 lines in 2 files changed: 2 ins; 5 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/27845.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27845/head:pull/27845

PR: https://git.openjdk.org/jdk/pull/27845