Refactor create_merge_request_dropdown.js

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The following discussion from !83426 (merged) should be addressed:

• @thomasrandolph started a discussion: (+1 comment)

  You could condense this a bit with something like this:

      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!

  ❔ What is this
  Type	Nitpick
  Blocker	No
  Tags	minor, verbosity, repetition
  Concern Level 1-10	2