DOC add Python 2.7 warning to recent whatsnew; include 23.3

[h-vetinari]

#18894 laid out:

We should add a big note on the top of each whatsnew that we are planning on dropping 2.7 support as of the end of 2018.

So far, this warning only appeared in v0.23.0, and can be easily missed these days. This PR adds them to the recent whatsnews, and also includes v0.23.3 in whatsnew.rst

[codecov]

Codecov Report

Merging #21944 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #21944   +/-   ##
=======================================
  Coverage   91.96%   91.96%           
=======================================
  Files         166      166           
  Lines       50323    50323           
=======================================
  Hits        46281    46281           
  Misses       4042     4042

Flag	Coverage Δ
#multiple	90.36% <ø> (ø)	⬆️
#single	42.22% <ø> (ø)	⬆️

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[jreback]

thanks