Convert issue board switcher to gitlab-ui dropdown components

What does this MR do?

Switch to gitlab-ui dropdown components (gitlab-ui!83 (merged)) for the issue board switcher.

This adds keyboard navigation and a "Switch board" header and removes the duplicate scrollbar.

What are the relevant issue numbers?

closes https://gitlab.com/gitlab-org/gitlab-ee/issues/8855

closes https://gitlab.com/gitlab-org/gitlab-ee/issues/9088

Screenshots

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• 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.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?