Drop support for v3-keys
- Those keys really should go away.
- gpg2.1 doesn't support them anymore.
- We can check fingerprints more accurately afterwards as we then know how long they must be.
Only concern: How to prevent gpg2.0 from importing them. (Or how to remove them instantly after importing them.) I would prefer to not allow these keys in the keyring and in the list of fingerprints, if we don't allow to use them for subscriptions. That could cause confusion.