Birthday input cast to datetime.date

[LaurentLM]

Reference issue

Fixes #13279.

What does this implement/fix?

Converts pd.Timestamp subject birthday input to datetime.date

pandas.Timestamp were allowed into info['subject_info']['birthday'] because they can be cast into a datetime.date. This was causing an issue when exporting to a FIFF file because the entry had not been converted to the right type.

Additional information

This seems to be specific to the birthday input, meas_date has the right checks to prevent pd.Timestamp inputs.