Add deprecation warning for `custom` Scan execution policy action

Why are we doing this work

The security policy pipeline execution action experiment was ended and the related code was removed in %17.3. We don't want to invalidate all policies that used this action, so we cannot remove it from the schema yet.

We should add a deprecation notice for policies using this action and show a banner with a link to the Pipeline execution policy documentation.

We can remove the action from the schema in %18.0.

Note: we should show a different banner than the one on the screenshot, a specific one to the custom scan action type.

Relevant links

• Discussion in the cleanup issue
• MR scheduled for %18.0 to remove the custom scan action from the schema

Non-functional requirements

• Documentation:
• Feature flag:
• Performance:
• Testing:

Implementation plan

• backend Extend ee/lib/security/scan_result_policies/deprecated_properties_checker.rb to account for the custom scan action
• backend Add deprecated_properties field to GQL ScanExecutionPolicyType
• frontend Parse deprecatedProperties from frontend for Scan execution policies
• frontend Add a new banner with a link to PEP documentation and show it if the new deprecated property is found

Verification steps

1. Create a project
2. Go to Secure -> Policies and create a new Scan execution policy and use the following YAML:

   type: scan_execution_policy
   name: Custom CI
   description: ''
   enabled: true
   rules:
     - type: pipeline
       branch_type: all
   actions:
     - scan: custom
       ci_configuration: |
         custom_job:
           script:
             - echo "Custom job!"

3. Go back to the policies list and verify that the policy has a deprecation icon and a banner is shown