Disallow license scans when policy is in warn-mode

What does this MR do and why?

This MR disallows the creation and use of license scan rules when the policy enforcement type is set to warn. As discussed in here, implementing warn mode for license policies has proven to be too complex for the current development cycle. Rather than implementing a partial solution that could lead to inconsistent behavior. This MR disallows the combination of license scan rules with warn enforcement type in the security policy schema.

References

• Implementation issue: #570791

How to set up and validate locally

1. Create a project
2. On the left sidebar, select Security & Compliance and Policies.
3. Select New Policy.
4. Select Merge request approval policy
5. Switch to .yaml mode
6. Paste the following policy

   approval_policy:
     - name: test
       description: ''
       enabled: true
       enforcement_type: warn
       policy_scope:
         projects:
           excluding: []
       rules:
         - type: license_finding
           match_on_inclusion_license: true
           licenses:
             allowed: []
           license_states: []
           branch_type: protected
       actions:
         - type: require_approval
           approvals_required: 1
           role_approvers:
             - maintainer
         - type: send_bot_message
           enabled: true
       approval_settings:
         block_branch_modification: true
         block_group_branch_modification: true
         prevent_pushing_and_force_pushing: true
         prevent_approval_by_author: true
         prevent_approval_by_commit_author: true
         remove_approvals_with_new_commit: true
         require_password_to_approve: false
       fallback_behavior:
         fail: closed

7. Select Create new project with the new policy
8. It should fail

MR acceptance checklist

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