Resolve "Some dropdowns have two scroll bars."

What does this MR do?

Adjusts specific max-height CSS rules to correspond to the new max-height of dropdowns of 312px.

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

No, but I did try to use flexbox to not have to continue to adjust max-heights if the spec for the dropdown changes, but it falls apart in IE11 since we need a height set for any elements that have min-height set, which defeats the purpose of having a dropdown with dynamic height... 😢

Why was this MR needed?

Because of a recent change to adjust the max height for all dropdowns to be 312px.

Screenshots (if relevant)

Before

After

One thing to note, is that the new height for dropdowns makes the dropdown for the Boards feature show only two options at a time, which doesn't seem optimal:

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes #43273 (closed)