Switch FX build to use JDK 12 as boot JDK

We need to switch to using JDK 12 as the boot JDK for building FX 13. This change blocks the following two issues:

JDK-8211308: Support HTTP/2 in WebView
JDK-8210508: Update JDK_DOCS property to point to JDK 12 docs

Note that this will not change the minimum JDK needed to build FX, which remains at JDK 11. It also will not bump the class file format for FX classes. We will still be able to run JavaFX 13 with JDK 11 (at least for now).

Note the following limitations after the above fixes are integrated.

JDK-8210508 -- When building with JDK 11 or 11.0.1, the docs will have broken links once this fix is integrated. Using JDK 11.0.2 will work

JDK-8211308 -- When running with JDK 11.*, WebView will fall back to HTTP/1