Proposal: Bump minimum JDK version for JavaFX 23 to JDK 21

[Kevin Rushforth]

Based on feedback, I've moved the PR [1] for this enhancement [2] back 
to 'rfr'. A Provisional CSR [2] is also ready for review (by one or more 
people with a "Reviewer" role in the project).

Thank you.

-- Kevin

[1] https://github.com/openjdk/jfx/pull/1370
[2] https://bugs.openjdk.org/browse/JDK-8321603
[3] https://bugs.openjdk.org/browse/JDK-8325993