Make it possible to actually use (e.g.) -Dsurefire.toolchain.version=21 to set the Java version for tests, and do so in CI. (The property value still _defaults_ to the JDK that you use to run Maven, so passing the flag is not _necessary_.)

Make it possible to actually use (e.g.) -Dsurefire.toolchain.version=21 to set the Java version for tests, and do so in CI. (The property value still _defaults_ to the JDK that you use to run Maven, so passing the flag is not _necessary_.) #4109