DOC: clean-up 0.23.1 whatsnew
I added a "Fixed regressions" section and moved some over there. @TomAugspurger does that look ok to you? (then I'll maybe merge to the other PRs can add it there)

Also mad the bug fixes subsections into plain text to make it a bit less "heavy" (given the only few entries per section).
@MeeseeksDev hello