Remove Bootstrap color variables

Split off from #2821 (comment 2206089141):

Once we don't need any of the variables in src/vendor/bootstrap/scss/_variables.scss anymore, we can do one breaking change in the library and remove them. We'd have to make sure that none of the other projects need the variables. If they do, we'd need to migrate them, or vendor them again. Uff, that sounds like a lot, right? So, this issue is specifically talking about design tokens. As a first step we could check whether there are color values in _variables.scss which we could change directly. I'd fear though that this is double work in the end though then.

This needs refinement. It's not clear yet how best to proceed.