to_datetime() throws a warning about UDFs

[ChuckConnell]

The first time I call to_datetime() in a Databricks notebook, it throws the warning: In Python 3.6+ and Spark 3.0+, it is preferred to specify type hints for pandas UDF instead of specifying pandas UDF type, which will be deprecated in the future releases. See SPARK-28264 for more details.

But this may be an incorrect warning, since to_datetime() does not seem to relate to SPARK-28264.

The warning does not happen when the same code is run again in the same notebook session.

This issue is partly addressed in #2044, but is not really answered there.