[jfx20] RFR: 8304359: Create release notes for JavaFX 20

[Kevin Rushforth]

On Thu, 16 Mar 2023 16:53:25 GMT, Kevin Rushforth <kcr at openjdk.org> wrote:

> Release notes for JavaFX 20, including one important change to note (the requirement of JDK 17 as the minimum needed to run JavaFX 20) and the list of enhancements and bugs fixed in this release.
> 
> Note: This PR is targeted to `jfx20`.

While reviewing the changes, I realized I forgot to include the release note for [FXML JavaScript Engine Disabled by Default](https://github.com/openjdk/jfx/blob/jfx19/doc-files/release-notes-19.0.2.md#fxml-javascript-engine-disabled-by-default) from the January CPU release notes. I'll update shortly.

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PR: https://git.openjdk.org/jfx/pull/1060