RFR: Merge jdk20

[Jesper Wilhelmsson]

Forwardport JDK 20 -> JDK 21

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

Commit messages:
 - Merge remote-tracking branch 'jdk20/master' into Merge_jdk20
 - 8298785: gc/TestFullGCCount.java fails with "invalid boolean value: `null' for expression `vm.opt.ExplicitGCInvokesConcurrent'"
 - 8298277: Replace "session" with "scope" for FFM access

The merge commit only contains trivial merges, so no merge-specific webrevs have been generated.

Changes: https://git.openjdk.org/jdk/pull/11683/files
  Stats: 132 lines in 20 files changed: 0 ins; 0 del; 132 mod
  Patch: https://git.openjdk.org/jdk/pull/11683.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11683/head:pull/11683

PR: https://git.openjdk.org/jdk/pull/11683