Use ALL_PROTECTED_BRANCHES text in rule editor for scan result policies

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR updates the Rule Mode UI for Scan Result Policies to allow users to select All protected branches instead of All branches. This is more accurate as it is consistent with the functionality that actually exists on the backend.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

1. As a project Owner, navigate to Security & Compliance > Policies
2. Create a new Scan Result Policy
3. Observe that by default the branch selection dropdown says All protected branches instead of All branches
4. Create a merge request for the new scan result policy
5. Merge in the merge request
6. Navigate back to your original project and navigate back to the Security & Compliance > Policies page
7. Click on your new policy and confirm that the text in the sidebar says all protected branches instead of all branches
8. Navigate to Settings > Merge Requests and scroll down to the Merge Request Approvals > Security Approvals section.
9. Confirm that the Target Branch column says All protected branches instead of All branches

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.