Update bootstrap to 4.3.1

What does this MR do?

Update bootstrap 4.3.1. This is needed for a future bootstrap-vue upgrade and also introduces a santizer for popovers. Thes default popover sanitzer had to be extended to accomodate for our popover usage.

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/52091

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer