[jfx21] RFR: 8316135: Create release notes for JavaFX 21

[Kevin Rushforth]

This is a necessary backport of the JavaFX 21 release notes to the `jfx21` branch. We will _not_ tag `jfx21` nor rebuild JavaFX 21 as a result of this commit.

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Commit messages:
 - Backport fa73e0d247dcadc8f80586c52ba7f92f3bb7eadb

Changes: https://git.openjdk.org/jfx/pull/1243/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1243&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8316135
  Stats: 153 lines in 1 file changed: 153 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jfx/pull/1243.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1243/head:pull/1243

PR: https://git.openjdk.org/jfx/pull/1243