As an integral part of forking the jfx20 branch, we need to change the release version to 21 in the master branch for JavaFX 21.

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

.jcheck/conf
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.