Fix `timestamptz` vs `timestamp` mismatch for `previous_api_key_expiration_datetime`

datetime.now(timezone.utc) produces timezone-aware datetimes (timestamptz), but the previous_api_key_expiration_datetime column is defined as DateTime() (naive, no timezone). CockroachDB v20.1 strictly rejects this type mismatch, while v23.2+ silently casts it, which is why this was not caught in development.

Switch to datetime.utcnow() to produce naive UTC datetimes, consistent with the rest of the codebase.

Assisted-by: Claude Code Signed-off-by: Miroslav Vadkerti mvadkert@redhat.com