RFR: 8253123: Switch FX build to use JDK 15 as boot JDK

[Kevin Rushforth]

Updates the boot JDK used to build JavaFX 16 to JDK 15. The minimum remains at JDK 11.

Note to reviewers: the build number for JDK 15 GA is the same as JDK 14 GA, so that property is intentionally unchanged
(I didn't forget to update it).

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Commit messages:
 - 8253123: Switch FX build to use JDK 15 as boot JDK

Changes: https://git.openjdk.java.net/jfx/pull/306/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=306&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253123
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jfx/pull/306.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/306/head:pull/306

PR: https://git.openjdk.java.net/jfx/pull/306