BUG: concat warning bubbling up through str.cat

[h-vetinari]

This was an unexpected realisation from #23723 -- the change of pd.concat in #20613 to add sort and raise a warning if it's not specified was not caught in the .str.cat code, because it was being shadowed (apparently in all relevant tests) by another FutureWarning that's being raised if join is not specified (from #20347).

This PR only adds the kwarg to the .str.cat internals where necessary.

Example from #23723:

>>> import pandas as pd
>>> s2 = pd.Series(['a', 'b', 'c', 'd'], index=['a', 'b', 'c', 'd'])
>>> u2 = pd.Series(['b', 'd', 'a', 'c'], index=['b', 'd', 'a', 'c'])
>>> s2.str.cat([pd.Index(['d', 'c', 'b', 'a']), u2, u2.values], na_rep='-', join='left')
[...] FutureWarning: Sorting because non-concatenation axis is not aligned. A future version
of pandas will change to not sort by default.
To accept the future behavior, pass 'sort=False'.
To retain the current behavior and silence the warning, pass 'sort=True'.
a    aaab
b    bbbd
c    ccca
d    dddc
dtype: object

[pep8speaks]

Hello @h-vetinari! Thanks for submitting the PR.

• There are no PEP8 issues in the file pandas/core/strings.py !

[h-vetinari]

Aaaaaand, another flaky hypothesis failure.

[codecov]

Codecov Report

Merging #23725 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #23725   +/-   ##
=======================================
  Coverage   92.25%   92.25%           
=======================================
  Files         161      161           
  Lines       51383    51383           
=======================================
  Hits        47404    47404           
  Misses       3979     3979

Flag	Coverage Δ
#multiple	90.64% <100%> (ø)	⬆️
#single	42.31% <0%> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/core/strings.py	98.58% <100%> (ø)	⬆️
pandas/plotting/_core.py	83.63% <0%> (ø)	⬆️
pandas/core/indexing.py	93.87% <0%> (ø)	⬆️
pandas/core/generic.py	96.82% <0%> (ø)	⬆️
pandas/core/arrays/categorical.py	95.34% <0%> (ø)	⬆️
pandas/core/window.py	96.4% <0%> (ø)	⬆️
pandas/core/arrays/datetimes.py	98.47% <0%> (ø)	⬆️

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[jreback]

thanks!