Resolve "Projects \/ menu displays scrolls bar for short period of time"

What does this MR do?

Resolves overflow issue with projects dropdown menu due to lack of specificity in its selector.

Are there points in the code the reviewer needs to double check?

If the relocation of the .projects-dropdown-menu rules makes sense. Part of the issue is due to the specificity of the selectors.

While it makes sense to include it only in the header SCSS, you would have to add .show to the selector. And since those selectors already exist in dropdowns.scss, it seemed to make more sense to add the specific case there, though it may be at the cost of a little extra CSS.

For what it's worth, this dropdown will be made re-usable for Groups in https://gitlab.com/gitlab-org/gitlab-ce/issues/36234.

Screenshots

Before

After

Does this MR meet the acceptance criteria?

• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
• Conform by the style guides
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #47158 (closed)