[jfx11u] RFR: 8240539: Upgrade gradle to version 6.3

[Kevin Rushforth]

On Fri, 7 May 2021 14:35:48 GMT, Ambarish Rapte <arapte at openjdk.org> wrote:

> jfx11u uses gradle 5.3 for build, it needs to be upgraded to gradle 6.3 same as jfx mainline.
> This PR backports three fixes:
> 
> 1. [JDK-8240539](https://bugs.openjdk.java.net/browse/JDK-8240539): Upgrade gradle to version 6.3
> 2. [JDK-8232063](https://bugs.openjdk.java.net/browse/JDK-8232063): Upgrade gradle to version 6.0 and
> 3. [JDK-8226754](https://bugs.openjdk.java.net/browse/JDK-8226754): FX build fails using gradle 5.6+ or 6
> 
> All the changes backport cleanly.

Looks good.

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Marked as reviewed by kcr (Lead).

PR: https://git.openjdk.java.net/jfx11u/pull/7