After the jfx14 branch is forked from jfx, we need to change the release version to 15 in the master branch for OpenJFX 15.

We need to update the release values in the following file:

build.properties

Additionally, since this bumps the major version we need to update the following unit test:

modules/javafx.base/src/test/java/test/com/sun/javafx/runtime/VersionInfoTest.java

See https://github.com/openjdk/jfx/blob/master/UPDATING-VERSION.md for details.