Update default value for `copy` parameter of `DataFrame.tz_convert` method
The default value is in accordance with the method's [documentation](https://pandas.pydata.org/docs/reference/api/pandas.DataFrame.tz_convert.html).

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Yes agreed, this is supposed to be None. This defaults to True in the non CoW case.  @jbrockmendel @phofl Is it worth updating the documentation for these methods then? As a user, this behavior of the functions is not obvious to me.

<img width="525" alt="image" src="https://github.com/pandas-dev/pandas/assets/45976948/bf899860-844e-40fb-a2eb-81d0accc584e">
