Prepare breaking change of Bootstrap in gitlab-ui

The dependency gitlab/ui now vendors Bootstrap. Although it contains the files now, it does not export them yet. First, we need to prepare this project to match the criterias of the upcoming breaking change. The tasks are summarized in this commit message.

In the scope of this issue, we need to verify that:

• Bootstrap version is pinned to 4.6.2.
• There is no import of bootstrap/scss/ (either replaced with @gitlab/ui/scr/vendor/bootstrap/scss or, preferred, refactored/migrated)
• There is no import of bootstrap/js/ (needs to be refactored/migrated if there are any. There is no replacement)