Fix `block_branch_modification` effects on protected branches

What does this MR do and why?

Fixes two bugs related to MR approval policies' block_branch_modification property:

• protected branches are only blocked from deletion when backed by a git ref, since PolicyBranchesService reads from Gitaly
• policy branches pattern matching behaves unexpectedly. Since our policies accept branch patterns, e.g. branches: [test-*], and protected branches are patterns, too, we have to treat one as strings for comparison, since we can't match a pattern against another pattern. Currently, we match policy branch spec (string) against the protected branch name (pattern). We want do the opposite

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Screenshots or screen recordings

Right-most Unprotect button:

Before	After

How to set up and validate locally

• Create a new group and a contained project

• On the project level:

  • Navigate to Settings > Repository and create the following branch protections:
    • develop
    • foo*
    • test-123
    • unrelated

• On the group level:

  • Navigate to Security > Policies and create the following MR approval policy:

type: approval_policy
name: Block protected branches
enabled: true
rules:
  - type: any_merge_request
    branches:
      - test-*
    commits: any
  - type: any_merge_request
    branches:
      - develop
    commits: any
  - type: any_merge_request
    branches:
      - foo*
    commits: any
  - type: any_merge_request
    branch_type: protected
    commits: any
actions:
  - type: require_approval
    approvals_required: 1
    role_approvers:
      - owner
approval_settings:
  block_branch_modification: true

• On the project level, navigate to Settings > Repository and verify that the UI controls allow only unrelated to be deleted.

• Verify none of the protected branches but unrelated can be deleted via API:

curl -X DELETE -H "PRIVATE-TOKEN: $GITLAB_TOKEN" "http://gdk.test:3000/api/v4/projects/$PROJECT_ID/protected_branches/develop"

Related to #456499 (closed)