RFR: 8283517: Update boot JDK to 18

[Ambarish Rapte]

On Fri, 25 Mar 2022 12:16:18 GMT, Kevin Rushforth <kcr at openjdk.org> wrote:

> JDK 18 has been released, so we should update the default boot JDK used to build JavaFX to JDK 18. This will not affect the minimum boot JDK version. I have confirmed that gradle 7.3 works with JDK 18, so we don't need to update gradle at this time.
> 
> I have done a full set of tests on all three platforms.

Marked as reviewed by arapte (Reviewer).

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PR: https://git.openjdk.java.net/jfx/pull/760