dtests not determining C* version correctly

In the course of CASSANDRA-14134, the means of extracting the C* version under test before starting a cluster became broken. This is necessary in cases where we want to gate values in cassandra.yaml based on version, so a couple of tests are affected. The specifics are that the global CASSANDRA_VERSION_FROM_BUILD was hardcoded to '4.0' and the ways in which the various tests use it have meant that it was undetected until now.

Also, the fixture_since which we use to implement the @since annotation is broken when a -cassandra-version is supplied, rather than -cassandra-dir, meaning testing against released versions from git isn't working right now.

Tests directly affected:

• auth_test.py - CASSANDRA-13985 added some gating of yaml props and additional checks on CQL results based on the build version. These failed on 3.11, which is how this issue was uncovered, but they're also broken on 2.2 on builds.apache.org
• user_functions_test.py - gates setting a yaml property when version < 3.0. Failing on 2.2.
• upgrade_tests - a number of these use the variable, but I don't think they're actually being run at the moment.
• repair_tests/repair_test.py, replace_address_test.py & thrift_test all use the global, but only to verify that the version is not 3.9. As we're not running CI for that version, no-one noticed.