Enable Libgit2 Managed Transport by Default

Managed Transport was introduced in `source-controller` `v0.22` and ever since has been kept behind a feature gate. 

This feature already behaves in more predictable ways than the unmanaged `libgit2` alternative.
Therefore, this feature should be enabled by default once all known issues are mitigated (#636) or there are good enough work-arounds for unmitigated issues.