DEPR: execute deprecations for str.cat in v1.0
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

This executes some deprecations from the 0.23/0.24 cycles, namely following the switch from `join=None` to `join='left'` (following #18657 / #20347), as well as the fact that list-likes *within* list-likes were deprecated (following #21950 / #22264).

Since this deprecation closes the loop to my first-ever pandas issue (#18657) and PR (#20347), I'll allow myself a bit of a digression here. After biting off a bit more than I could chew (after finding out just how many cases the method was supposed to support), things got a bit tight with the cut-off for `0.23.0rc`, and @TomAugspurger [quipped](https://github.com/pandas-dev/pandas/pull/20347#issuecomment-385686992):

> > @h-vetinari: Thanks for waiting with the cut-off! [...]

> @TomAugspurger: Right, we'll keep you on the hook for future maintenance of this :)

After #20347 #20790 #20842 #20845 #20923 #21330 #21950 #22264 #22575 #22652 #22721 (-->#23011 #23163 #23167 #23582 ... and several more issues/PRs around `.str`-accessor) #22722 #22725 #23009 (-->#23061 #23065) #23187 #23443 #23723 #23725 #24044 #24045 #26605 #26607 etc., I think I can say that I upheld my end of the bargain. :)