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) Update: Better IDEA! See below
• There is no import of bootstrap/js/ (needs to be refactored/migrated if there are any. There is no replacement)

Better idea for SCSS imports

Instead of touching SCSS files, we could instead configure the bundler to use gitlab-ui's node modules for sass:

(Thanks @leipert for the idea.) This would not help with the bootstrap/js/ imports though.