Raise a NotImplementedError in to_datetime when utc is passed (#13670)

While we figure out a solution for #13661, this PR ensures that we correctly raise a `NotImplementedError`.

Authors:
  - Ashwin Srinath (https://github.com/shwina)

Approvers:
  - Matthew Roeschke (https://github.com/mroeschke)

URL: #13670

python/cudf/cudf/core/tools/datetimes.py

@@ -144,6 +144,9 @@ def to_datetime(
     if yearfirst:
         raise NotImplementedError("yearfirst support is not yet implemented")
 
+    if utc:
+        raise NotImplementedError("utc is not yet implemented")
+
     if format is not None and "%f" in format:
         format = format.replace("%f", "%9f")