RFR: 8255714: Switch FX build to use JDK 15.0.1 as boot JDK

[Kevin Rushforth]

Bump the boot JDK used to build JavaFX to 15.0.1. This does not change the minimum boot JDK which remains at JDK 11.

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

Commit messages:
 - 8255714: Switch FX build to use JDK 15.0.1 as boot JDK

Changes: https://git.openjdk.java.net/jfx/pull/347/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=347&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8255714
  Stats: 11 lines in 2 files changed: 0 ins; 0 del; 11 mod
  Patch: https://git.openjdk.java.net/jfx/pull/347.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/347/head:pull/347

PR: https://git.openjdk.java.net/jfx/pull/347