RFR: 8236733: Change JavaFX release version to 15

[Kevin Rushforth]

JDK-8236733](https://bugs.openjdk.java.net/browse/JDK-8236733).

Bump the release version to 15. As noted in the JBS issue, I will wait until right after the `jfx14` fork to integrate this.

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Commits:
 - 50f38ab6: 8236733: Change JavaFX release version to 15

Changes: https://git.openjdk.java.net/jfx/pull/83/files
 Webrev: https://webrevs.openjdk.java.net/jfx/83/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8236733
  Stats: 4 lines in 2 files changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jfx/pull/83.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/83/head:pull/83

PR: https://git.openjdk.java.net/jfx/pull/83