Adjust board switcher dropdown selector

What does this MR do?

Make the board switcher CSS selector more specific so it does not apply to other dropdowns. This fixes the misalignment of dropdowns in the edit board modal.

What are the relevant issue numbers?

closes https://gitlab.com/gitlab-org/gitlab-ee/issues/10261

Screenshots

Before	After

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.
• 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?
• Security reports checked/validated by reviewer