RFR: 8279615: Change JavaFX release version to 19

[Kevin Rushforth]

Bump the version number of JavaFX to 19. I will integrate this immediately after forking the `jfx18` stabilization branch, which is scheduled for Thursday, January 13, 2021 at 16:00 UTC.

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

Commit messages:
 - 8279615: Change JavaFX release version to 19

Changes: https://git.openjdk.java.net/jfx/pull/707/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=707&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8279615
  Stats: 5 lines in 3 files changed: 0 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/jfx/pull/707.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/707/head:pull/707

PR: https://git.openjdk.java.net/jfx/pull/707