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

[Ambarish Rapte]

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.

-------------

Commit messages:
 - 8240539: Upgrade gradle to version 6.3
 - 8232063: Upgrade gradle to version 6.0
 - 8226754: FX build fails using gradle 5.6+ or 6

Changes: https://git.openjdk.java.net/jfx11u/pull/7/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx11u&pr=7&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8240539
  Stats: 31 lines in 6 files changed: 17 ins; 4 del; 10 mod
  Patch: https://git.openjdk.java.net/jfx11u/pull/7.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx11u pull/7/head:pull/7

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