CLN use pd.Timestamp for single element in converter
Only a single element is being parsed, and there's no way for a user to pass any options (like `dayfirst`) through `ax.set_xlim`, so might as well use `pd.Timestamp`.

Otherwise with https://github.com/pandas-dev/pandas/pull/49024 users might get a warning they wouldn't be able to do anything about

Example of test that hits this: `pytest pandas/tests/plotting/test_datetimelike.py -k test_time_change_xlim`
thx @MarcoGorelli 