RFR: JDK-8296149: Start of release updates for JDK 21

[Joe Darcy]

Usual start-of-release updates. Symbol updates in initial version reflect JDK 20 build 21.

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Commit messages:
 - Fix failed HotSpot regression tests.
 - Merge branch 'master' into JDK-8296149
 - Update symbol information to JDK 20 build 24.
 - Merge branch 'master' into JDK-8296149
 - Merge branch 'master' into JDK-8296149
 - Update symbols for JDK 20 b23.
 - Merge branch 'master' into JDK-8296149
 - Merge branch 'master' into JDK-8296149
 - Update symbols for JDK 20 build 22.
 - Update expected GA date.
 - ... and 1 more: https://git.openjdk.org/jdk/compare/105d9d75...5810345e

Changes: https://git.openjdk.org/jdk/pull/10924/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10924&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8296149
  Stats: 3653 lines in 69 files changed: 3608 ins; 1 del; 44 mod
  Patch: https://git.openjdk.org/jdk/pull/10924.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10924/head:pull/10924

PR: https://git.openjdk.org/jdk/pull/10924