Clean up breakpoint hack after Bootstrap v5.3

[ebroder]

Bootstrap stopped including the grid breakpoint sizes as variables in v5. There's a merged PR to re-introduce them that is scheduled to appear in v5.3, which isn't released yet. In the interim, we've manually added them in our SCSS. Once v5.3 is out and we have upgraded, we should drop that logic from _theme.scss.

(This is a followup to #1181)