gpg: Do not fail with an error for a "Note:" diagnostic

* g10/trustdb.c (validate_keys): Use log_info instead of log_error for
not found or expired UTKs.

Actually the not-found case used log_error for decades. The
semantically simialr expired case did thus the same. The actual
problem is for example in the import case where gpg exits with a
failure despite that a key validation was requested.

• GnuPG-bug-id: T7351