RFR: 8369433: G1: Replace G1MarkScope with NMethodMarkingScope

[Francesco Andreuzzi]

Replacing `G1MarkScope` with `NMethodMarkingScope`, since they have equivalent implementations.

Passes tier1 and tier2 (fastdebug).

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Commit messages:
 - replace

Changes: https://git.openjdk.org/jdk/pull/27697/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27697&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8369433
  Stats: 6 lines in 1 file changed: 1 ins; 4 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/27697.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27697/head:pull/27697

PR: https://git.openjdk.org/jdk/pull/27697