RFR: 8240539: Upgrade gradle to version 6.3

Wed Mar 25 21:25:59 UTC 2020

On Wed, 25 Mar 2020 21:20:37 GMT, Kevin Rushforth <kcr at openjdk.org> wrote:

> This upgrades the version of gradle used to build JavaFX to 6.3. The minimum version of gradle is not changed by the
> proposed fix, and remains at 5.3.
> I have run done a full build and test with gradle 6.3 on all three platforms. There are no new failures.
> 
> This will enable building JavaFX using JDK 14 as the boot JDK, although upgrading the boot JDK for our builds will be
> done in a separate pull request.

@arapte can you also review?

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PR: https://git.openjdk.java.net/jfx/pull/151