Limit supporte values of --org for verify-codeowners

[KyleFromNVIDIA]

The argument was already implicitly limited to these values due to how the code is structured, but codify the limitation in the arguments.

[jameslamb]

I haven't used this before, but from my read of https://docs.python.org/3/library/argparse.html#choices this would mean that values not in this list are rejected.

argument values will be checked, and an error message will be displayed if the argument was not one of the acceptable values

I'm fine with being strict for now until we have a known reason to support more orgs, but just would ask that you change the PR title (which will show up in the release notes) to reflect that that was the intent.

Something like "verify-codeowners: limit supported values of --org".

"add choices" sounds like we're expanding the set of available values, but the code is actually restricting the set of supported values.

[KyleFromNVIDIA]

I'm fine with being strict for now until we have a known reason to support more orgs

Other orgs were already not supported (at least for CI and packaging), since those would reference nonexistent entries in the dictionaries. This simply codifies the restriction.

I'll update the title.

[KyleFromNVIDIA]

/merge