RFR: 8236733: Change JavaFX release version to 15

[Ambarish Rapte]

On Thu, 9 Jan 2020 02:39:09 GMT, Kevin Rushforth <kcr at openjdk.org> wrote:

> JDK-8236733](https://bugs.openjdk.java.net/browse/JDK-8236733).
> 
> Bump the release version to 15. As noted in the JBS issue, I will wait until right after the `jfx14` fork to integrate this.

Looks good to me.

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Marked as reviewed by arapte (Reviewer).

PR: https://git.openjdk.java.net/jfx/pull/83