added 245px max height and overflow scroll-y to .grouped-pipeline-dropdown

What does this MR do?

It restricts the sub builds popup in the pipeline graph and makes it scrollable

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

none that I know of

Why was this MR needed?

To restrict crazy CI configurations from messing with the viewport scrolling.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #22295 (closed)