depr(python, rust!): Rename str.concat to str.join and update default delimiter

[stinodego]

This brings the method in line with list.join and arr.join, and Python's join method on strings.

Changes

• Deprecate Expr/Series method str.concat. Users should use str.join instead.

Example

Before

>>> s = pl.Series(["a", "b", "c"])
>>> s.str.concat()
shape: (1,)
Series: '' [str]
[
        "a-b-c"
]

After

>>> s.str.join()
shape: (1,)
Series: '' [str]
[
        "abc"
]

[codspeed-hq]

CodSpeed Performance Report

Merging #16790 will degrade performances by 17.2%

_{Comparing depr-str-concat (8204fb2) with main (1fbfa08)}

Summary

❌ 1 regressions
✅ 36 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	main	depr-str-concat	Change
❌	test_groupby_h2oai_q5	1.8 ms	2.1 ms	-17.2%

[codecov]

Codecov Report

Attention: Patch coverage is 94.44444% with 1 line in your changes missing coverage. Please review.

Project coverage is 81.34%. Comparing base (cee0e53) to head (8204fb2).
Report is 1 commits behind head on main.

Files	Patch %	Lines
py-polars/polars/expr/string.py	85.71%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16790      +/-   ##
==========================================
+ Coverage   81.32%   81.34%   +0.01%     
==========================================
  Files        1424     1424              
  Lines      187169   187173       +4     
  Branches     2698     2699       +1     
==========================================
+ Hits       152221   152253      +32     
+ Misses      34452    34423      -29     
- Partials      496      497       +1     

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