RFR: 8232063: Upgrade gradle to version 6.0

[Kevin Rushforth]

This PR upgrades the version of gradle used to build JavaFX to 6.0. In addition to keeping up to date, this will allow us to subsequently upgrade the boot JDK to JDK 13.

This change does the following:

1. Use ivy `patternLayout ...` instead of `layout "pattern" ...` and specify no metadata for ivy repositories. This was split out of [JDK-8226754](https://bugs.openjdk.java.net/browse/JDK-8226754), PR #9, because the replacement API is documented as unstable in gradle 5.x (although it works fine) and isn't needed until we actually upgrade to 6.0.

2. Bump the gradle version used by the build from 5.3 to 6.0.

Note that this does not change the minimum required version of gradle, which remains at 5.3.

I have compared the artifacts produced before and after this change and there are no differences.

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Commits:
 - 5b224e5d: 8232063: Upgrade gradle to version 6.0

Changes: https://git.openjdk.java.net/jfx/pull/33/files
 Webrev: https://webrevs.openjdk.java.net/jfx/33/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8232063
  Stats: 24 lines in 5 files changed: 15 ins; 0 del; 9 mod
  Patch: https://git.openjdk.java.net/jfx/pull/33.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/33/head:pull/33

PR: https://git.openjdk.java.net/jfx/pull/33