This PR introduces several fixes to various IT tests, related to the use and misuse of the version identifier for the start cluster:

    wherever we can, we replace of versions in test code with features
    where we can't, we make sure we use the actual stack version (the one provided by -Dtests.bwc.main.version and not the bogus "0.0.0" version string)
    when requesting the cluster version we make sure we do use the "unresolved" version identifier (the value of the tests.old_cluster_version system property e.g. 0.0.0 ) so we resolve the right distribution

These changes enabled the tests to be used in BC upgrade tests (and potentially in serverless upgrade tests too, where they would have also failed)

Relates to ES-12010

Precedes elastic#128614, elastic#128823 and elastic#128983