Change JavaFX release version to 18

After the jfx17 branch is forked from jfx, we need to change the release version to 18 in the master branch for OpenJFX 18.

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.