RFR: 8306630: Bump minimum boot jdk to JDK 21

[Mikael Vidstedt]

With the JDK 21 GA out it's time to bump the minimum boot JDK version for mainline/JDK 22.

Testing: tier1-5, GHA (in progress)

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Commit messages:
 - 8306630: Bump minimum boot jdk to JDK 21

Changes: https://git.openjdk.org/jdk/pull/15934/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15934&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8306630
  Stats: 9 lines in 3 files changed: 0 ins; 0 del; 9 mod
  Patch: https://git.openjdk.org/jdk/pull/15934.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15934/head:pull/15934

PR: https://git.openjdk.org/jdk/pull/15934