RFR: Merge jdk20

[Jesper Wilhelmsson]

Forwardport JDK 20 -> JDK 21

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Commit messages:
 - Merge remote-tracking branch 'jdk20/master' into Merge_jdk20
 - 8301989: new javax.swing.text.DefaultCaret().setBlinkRate(N) results in NPE
 - 8301863: ObjectInputFilter example incorrectly calls rejectUndecidedClass

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

Changes: https://git.openjdk.org/jdk/pull/12484/files
  Stats: 69 lines in 3 files changed: 50 ins; 9 del; 10 mod
  Patch: https://git.openjdk.org/jdk/pull/12484.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12484/head:pull/12484

PR: https://git.openjdk.org/jdk/pull/12484