refactor(rust): Move replace_time_zone to polars-core

[mcrumiller]

This is needed to resolve #25674.

---

In order to fix #25674, we must update the cast from datetime to date, and replace_time_zone, which currently resides in the polars_ops crate, needs to be accessible from the cast function. Once this is in, the fix for that issue will be fairly trivial (add replace_time_zone to the cast).

There are no logical changes, this PR makes replace_time_zone a proper method of DatetimeChunked.

[codecov]

Codecov Report

❌ Patch coverage is 98.13953% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.73%. Comparing base (cb992fc) to head (27147a7).

Files with missing lines	Patch %	Lines
crates/polars-expr/src/dispatch/datetime.rs	92.10%	3 Missing ⚠️
crates/polars-expr/src/dispatch/temporal.rs	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #25680      +/-   ##
==========================================
- Coverage   80.50%   79.73%   -0.77%     
==========================================
  Files        1771     1770       -1     
  Lines      243327   243326       -1     
  Branches     3037     3037              
==========================================
- Hits       195883   194017    -1866     
- Misses      46662    48527    +1865     
  Partials      782      782              

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