Follow-up: Change protected branches selector to accept a single selected branch rather than an array

The following discussion from !92519 (merged) should be addressed:

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

  question: why do we need to pass selectedBranches as an array since we can only select a single branch with this component?

Implementation plan

1. Update protected_branches_selector.vue's selectedBranches prop to be a string instead of an array.
   • Update all use cases of the component.
2. Update specs.

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