RFR: 8316135: Create release notes for JavaFX 21

[Kevin Rushforth]

Release notes for JavaFX 21, including four important changes, and the list of enhancements and bugs fixed in this release.

I plan to integrate this on Monday, Sep 18th, and backport it to `jfx21` in time for Tuesday's release of JavaFX 21.

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Commit messages:
 - 8316135: Create release notes for JavaFX 21

Changes: https://git.openjdk.org/jfx/pull/1239/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1239&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8316135
  Stats: 152 lines in 1 file changed: 152 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jfx/pull/1239.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1239/head:pull/1239

PR: https://git.openjdk.org/jfx/pull/1239