Refactor to use listbox-item in MR add approval rule drawer

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What does this MR do and why?

References

Related Issue - #498296 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Go to any Project.
2. From sidebar go to settings > merge requests > Go to Merge request Approvals
3. Click on Add approval rule button.
4. Look for Target branch dropdown.

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #498296 (closed)