Migrates gitlab-sshd main file to labkit v2

Migrating gitlab-shell over to the v2 log package in LabKit is a significant undertaking - because of this, I've opted to break this down into a series of smaller chunks that will allow us to deliver this incrementally, and more importantly, more safely.

This MR mainly touches the sad path of gitlab-shell. This acts as a safer litmus test that the v2 logging package will be functionally equivalent to that of the current setup. If we've missed anything, then the impact of such a miss will be limited and negligible.

One thing to consider - we can opt for an even safer approach by emitting duplicate log lines and doing a comparison - I'll leave it up to the reviewer to decide if we want to go down such a path.