CLN: Old timezone issues

[mroeschke]

• closes Indexing datetime index over summer/winter time change gives incorrect result #21846
• closes BUG: DatetimeIndex.unique shifts tz-aware dates #21737
• closes Hitting AmbiguousTimeError when exiting DST using .loc #20724
• closes possible bug when using datetime.replace() on a tz-aware pd.DatetimeIndex #18785
• closes Setting values on non-existing datetime index raises exception only when index has DST changes in range #18308
• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

[codecov]

Codecov Report

Merging #22201 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #22201   +/-   ##
=======================================
  Coverage   92.06%   92.06%           
=======================================
  Files         169      169           
  Lines       50694    50694           
=======================================
  Hits        46671    46671           
  Misses       4023     4023

Flag	Coverage Δ
#multiple	90.47% <ø> (ø)	⬆️
#single	42.32% <ø> (ø)	⬆️

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[jreback]

thanks @mroeschke nice cleaning these old issues!