[#27697] Make the arrow icons consistent in dropdown

What does this MR do?

Make the arrow icons consistent in dropdown from https://gitlab.com/gitlab-org/gitlab-ce/issues/27697

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

• Was the original plan to make it all (sitewide) dropdowns/select arrow to appear as fa-chevron-down? I am thinking in terms of whether to refactor all the select tags into a single place? By looking at this code, I noticed 3 ways (there might be more) of using dropdowns - HTML Select Tag, Select2, and VueJS Dropdown.
• Second opinion on whether this approach is considered 'hacky' would be helpful.

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Tests
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/27697

Closes #27697 (closed)