RFR: 8316135: Create release notes for JavaFX 21 [v2]

[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.

Kevin Rushforth has updated the pull request incrementally with one additional commit since the last revision:

  Add missing bugs JDK-8305885 and JDK-8304290

-------------

Changes:
  - all: https://git.openjdk.org/jfx/pull/1239/files
  - new: https://git.openjdk.org/jfx/pull/1239/files/d663aee6..56120aec

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1239&range=01
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1239&range=00-01

  Stats: 2 lines in 1 file changed: 2 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