RFR: 8232064: Switch FX build to use JDK 13.0.1 as boot JDK

[Kevin Rushforth]

Now that we have switched to using gradle 6 we can switch to using JDK 13 as the boot JDK for JavaFX 14 builds. The latest JDK 13 update release is JDK 13.0.1.

This will not change the minimum JDK version needed to build or run JavaFX, which remains at 11. We will continue to generate class files using `-source 11 -target 11`.

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Commits:
 - 741414f1: 8232064: Switch FX build to use JDK 13.0.1 as boot JDK

Changes: https://git.openjdk.java.net/jfx/pull/51/files
 Webrev: https://webrevs.openjdk.java.net/jfx/51/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8232064
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jfx/pull/51.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/51/head:pull/51

PR: https://git.openjdk.java.net/jfx/pull/51