Disallow with_time_zone on tz-naive

[MarcoGorelli]

Problem description

Breaking this off from #6561

with_time_zone on a tz-naive converts as if it were starting from UTC:

In [8]: tz_naive
Out[8]:
shape: (1,)
Series: '' [datetime[μs]]
[
        2020-01-01 03:00:00
]

In [9]: tz_naive.dt.with_time_zone('America/Barbados')
Out[9]:
shape: (1,)
Series: '' [datetime[μs, America/Barbados]]
[
        2019-12-31 23:00:00 AST
]

I think this is rarely what people are actually looking for - instead, they probably wanted tz_localize / cast_time_zone:

In [10]: tz_naive.dt.tz_localize('America/Barbados')
Out[10]:
shape: (1,)
Series: '' [datetime[μs, America/Barbados]]
[
        2020-01-01 03:00:00 AST
]

So, I think it would be more user-friendly to raise an error, leading people towards cast_time_zone

---

Having looked into this further, I think this might have been the original intention in https://github.com/pola-rs/polars/pull/4645/files, and that with_time_zone on tz-naive might only be working accidentallly:

polars/polars/polars-lazy/polars-plan/src/dsl/dt.rs

Lines 77 to 79 in afc55f5

	dt => Err(PolarsError::ComputeError(
	format!("Series of dtype {dt:?} has got no time zone").into(),
	)),

@ritchie46 could you confirm please? (and apologies if I misread your original intention!)