Resolve "Protected branch form should have sane defaults for dropdowns"

What does this MR do?

Adds defaults based on the previous selection for the merge and push dropdowns for the protected branches form in the repository settings

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

Probably the gl_dropdown changes

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #32163 (closed)