Feature request: Run post-flight checks only

[drmaciej]

Use case

I like the pre-flight check, but I do not want to use this tool for the actual upgrade - I use my existing codebase for that. I'd like to run post-flight checks in isolation, without the upgrade.

Solution/User Experience

A --postflight CLI toggle which leads to invocation of pre_flight_checks with preflight=False; no other action is performed.
I am aware of #103, but there seems to be no ETA on it, so I wonder if --postflight might be a good interim solution.

Alternative solutions

No response