HEADS-UP: bumping the boot JDK to JDK 12 for JavaFX 13

[Kevin Rushforth]

Now that JDK 12 has been released, I propose to bump the boot JDK used 
to build JavaFX 13 in production builds, and by the CI builds on GitHub, 
to JDK 12.

This will *not* change the minimum JDK required to build or run JavaFX 
13. However, there will be a couple of limitations when using JDK 11 to 
build or run.

Also note, that while this isn't a formal RFR, since no changeset will 
go into the HG repo, there is a GitHub PR #410 [1] under review for the 
Travis and Appveyor CI test builds (which exclusively touches 
GitHub-only files) if anyone wants to review it. The production setup is 
handled outside of the repo.

See JDK-8221097 [2] for more details on this.

-- Kevin

[1] https://github.com/javafxports/openjdk-jfx/pull/410
[2] https://bugs.openjdk.java.net/browse/JDK-8221097