Fix MR target branch select dropdown placement cut-off

What does this MR do?

• Fix target branch select dropdown cut-off when branch names are too long

EE MR: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1748

When we have enough space

When we don't have enough space

When branch names not long enough

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

• select2 seems to have some faulty logic with dropdownAutoWidth and not enough options to fix this succinctly(with a placement option for example).

This isn't a regression in the truest sense. But we are going to "call it a regression because the feature went from usable to almost unusable. -- @brycepj, https://gitlab.com/gitlab-org/gitlab-ce/issues/31271#note_28176095"

Why was this MR needed?

When the project has long branch names, the target branch selector on the edit MR page is placed off the screen (outside of the viewport).

Does this MR meet the acceptance criteria?

• 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?

Closes #31271 (closed)