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

[Andy Goryachev]

On Mon, 18 Sep 2023 15:39:18 GMT, Kevin Rushforth <kcr at openjdk.org> wrote:

> 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.

looks good.  will this create a merge conflict down the road?

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Marked as reviewed by angorya (Reviewer).

PR Review: https://git.openjdk.org/jfx/pull/1243#pullrequestreview-1631480621