Add search field to “Target branch” dropdown in approval rule modal

Originally in scope for #460 (closed), this part was not implemented in !22749 (closed) (see MR thread, !22749 (comment 269540872)) and so this issue aims to resolve the resulting UX debt.

When selecting a target branch to scope the approval rule, we're using a single-select dropdown. In !22749 (closed) the dropdown was implemented without a search field. However, the search field is needed for a correct UX that matches our Pajamas guidelines for Dropdowns:

If there are more than 10 elements in the list, it may be necessary to include a filter box.

In this case the number of available options is unknown, there could be 1, 20, 50, we don't know.

The dropdown should then feature a search field, like so: