RFR: 8321603: Bump minimum JDK version for JavaFX to JDK 21

[Kevin Rushforth]

This PR bumps the minimum version of the JDK needed to run JavaFX to **JDK 21**.

In order for JavaFX to be able to use newer JDK features, such as code snippets (in API docs), record patterns, pattern matching for switch statements, and so forth, we need to increase the minimum version of the JDK required run the latest JavaFX. Additionally, there is an ongoing cost to keeping JavaFX buildable and runnable on older versions of Java, and little reason to continue to do so.

This is targeted to JavaFX 23, and must not be backported to any earlier version. This needs a CSR and a release note.

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Commit messages:
 - 8321603: Bump minimum JDK version for JavaFX to JDK 21

Changes: https://git.openjdk.org/jfx/pull/1370/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1370&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8321603
  Stats: 8 lines in 3 files changed: 0 ins; 3 del; 5 mod
  Patch: https://git.openjdk.org/jfx/pull/1370.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1370/head:pull/1370

PR: https://git.openjdk.org/jfx/pull/1370