Improve borders of input group buttons

Fixes #11400.

**Tip:** using `:first-child` and `:last-child` isn't very well for this. `:first-of-type` and `:last-of-type` is much better. Try with dropdown button. It isn't the last child (but last button), so it will loose the border-radius if use `:not(...):not(:last-child)`.

### Don't merge so far!