DOC add Python 2.7 warning to recent whatsnew; include 23.3
#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`
