RFR: 8306630: Bump minimum boot jdk to JDK 21

[Iris Clark]

On Tue, 26 Sep 2023 21:22:07 GMT, Mikael Vidstedt <mikael at openjdk.org> wrote:

> 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)

Marked as reviewed by iris (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/15934#pullrequestreview-1645323993