Partially addresses #47424.

Additionally we're delegating missing method calls on GpgKeySubkey to
GpgKey since most of the info required when verifying a signature is
found on GpgKey which is the parent of GpgKeySubkey

The content returned by #key is important when veryfying the signature,
so given we don't want to repeat it in the database for GPG subkeys we
need to delegate it to the parent.

In line with other changes in GitLab, make email address validation
properly case insensitive. The email address in the commit may be in
any case, so it needs downcasing to match the address stored in GitLab
for the user. Without this change the comparison fails and commits are
not marked as verified.

See #37009.

the updated verification of a gpg signature requires the committer's
email to also match the user's and the key's emails.

when omitting the end part of the key ('-----END PGP PUBLIC KEY
BLOCK-----') the error message was not about the key anymore, but about
the missing fingerprint and primary_keyid, which was confusing for the
user.
the new validation checks that the end also matches the expected format.

other than just removing a key, which doesn't affect the verified state
of a commit, revoking a key unverifies all signed commits.