Update bootstrap-vue to latest version

Styling Differences:

• Dropdowns don't have automatic focus anymore on first item (which looks like a bug fix)
• Dropdown Items render a li now automatically around it when they are rendered outside of an dropdown (which is only the case in our examples)
• Form + Form Controls different paddings (as they don't have form-control or specific form-control-sm anymore applied to them)
• Modal background problem (we were missing the vue-bootstrap css styles that set the opacity on the backdrop)

I tested all components in storybook and they looked fully functional. Here are the 10 failing snapshots that were updated https://gitlab.com/gitlab-org/gitlab-ui/-/jobs/240368033

Closes #188 (closed)