snazy CASSANDRA-15835: Use the right Java version + JAVA_HOME when building…

… C* from source (#714)

Also:
* Add a new "version source type" `clone:/path/to/local/cassandra/clone` to re-enable upgrade-dtests for dev-CI and local upgrade-dtest runs after CASSANDRA-14420 + CASSANDRA-14421
* Introduce new node startup logfiles for stdout+stderr
* Use `./gradlew` instead of `ant` to build C*, if `gradlew` is present