RFR: 8296153: Bump minimum boot jdk to JDK 20

[Mikael Vidstedt]

With the JDK 20 GA out it's time to bump the minimum boot JDK version for mainline/JDK 21.

Testing: tier1-5, GHA (the Windows build successfully downloads the JDK but then fails for unrelated reasons, and one seemingly unrelated Vector API test failure in tier 3)

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Commit messages:
 - 8296153: Bump minimum boot jdk to JDK 20

Changes: https://git.openjdk.org/jdk/pull/13565/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13565&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8296153
  Stats: 9 lines in 3 files changed: 0 ins; 0 del; 9 mod
  Patch: https://git.openjdk.org/jdk/pull/13565.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13565/head:pull/13565

PR: https://git.openjdk.org/jdk/pull/13565