Resolves "Create new board section should be sticky"

What does this MR do?

Updates the dropdown code to make the dropdown footer be sticky using flexbox, fixing the issue of the Create New Board section be sticky, as described in https://gitlab.com/gitlab-org/gitlab-ce/issues/47721.

Note: this fix can be made more specific to the Issues Board context, but trying to see if this fix can be made safely and be beneficial to every place it's used.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/47721

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?