RFR: 8253123: Switch FX build to use JDK 15 as boot JDK

[Johan Vos]

On Tue, 15 Sep 2020 16:04:29 GMT, Kevin Rushforth <kcr at openjdk.org> wrote:

> Updates the boot JDK used to build JavaFX 16 to JDK 15. The minimum remains at JDK 11.
> 
> Note to reviewers: the build number for JDK 15 GA is the same as JDK 14 GA, so that property is intentionally unchanged
> (I didn't forget to update it).

Marked as reviewed by jvos (Reviewer).

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PR: https://git.openjdk.java.net/jfx/pull/306