[jfx11u] RFR: 8280489: Change JavaFX release version in jfx11u to 11.0.15

[Kevin Rushforth]

Bump release version in `jfx11u` repo to 11.0.15 for the start of a new release.

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

Commit messages:
 - 8280489: Change JavaFX release version in jfx11u to 11.0.15

Changes: https://git.openjdk.java.net/jfx11u/pull/69/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx11u&pr=69&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8280489
  Stats: 2 lines in 2 files changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jfx11u/pull/69.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx11u pull/69/head:pull/69

PR: https://git.openjdk.java.net/jfx11u/pull/69