Run all of verifyproblem for 2023-07 problems (even if we do not support many 2023-07 features)

[gkreitz]

Now runs through all of verifyproblem for 2023-07 problems. Most parts have not been updated to follow the standard yet, but I felt we're at a stage where it makes more sense to just try to do validation rather than exit after verifying just the config. I did quick testing locally with hello and different by just updating problem.yaml and renaming output_validators and problem_statements, and those now pass successfully. I'm unsure if they are actually valid 2023-07 packages with just those simple fixes.

Adds a warning about 2023-07 support being incomplete.

Also a bugfix where I'd messed up our formatting of pydantic errors, giving very ugly messages on problem.yaml errors.