Refactor create_merge_request_dropdown.js
<!--IssueSummary start-->
<details>
<summary>
Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards.
</summary>
- [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=356603)
</details>
<!--IssueSummary end-->
The following discussion from !83426 should be addressed:
- [ ] @thomasrandolph started a [discussion](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/83426#note_884571667): (+1 comment)
> You could condense this a bit with something like this:
>
> ```suggestion:-6+0
> this.unavailableButtonText.textContent = isLoading
> ? __('Checking branch availability...')
> : __('New branch unavailable');
>
> this.unavailableButtonSpinner.classList.toggle('gl-display-none', !isLoading);
> ```
>
> To be clear, this is only a very small verbosity/succinctness/repetition improvement. If you prefer the `if`/`else` format, you can safely ignore this and it's totally fine!
>
> |:grey_question: [What is this](https://gitlab.com/thomasrandolph/reviews/wikis/Review-Metadata-Table)||
> |-|-|
> | Type | **Nitpick** |
> | Blocker | No |
> | Tags | _minor_, _verbosity_, _repetition_ |
> | Concern Level 1-10 | 2 |
issue