Allow maintainers to set MR approval settings

As part of the new MR approval setting functionality
we accidentally removed project maintainers access
to set the settings.

This was discovered during flag roll-out and this
commit fixes the issue so rollout can continue.

Changelog: fixed
EE: true

ee/app/policies/ee/project_policy.rb

@@ -392,7 +392,7 @@ module EE
 
       rule { auditor | can?(:developer_access) }.enable :add_project_to_instance_security_dashboard
 
-      rule { (admin | owner) & group_merge_request_approval_settings_enabled }.policy do
+      rule { (admin | maintainer) & group_merge_request_approval_settings_enabled }.policy do
         enable :admin_merge_request_approval_settings
       end
     end

ee/spec/policies/project_policy_spec.rb

@@ -1414,7 +1414,7 @@ RSpec.describe ProjectPolicy do
       :reporter   | true  | false
       :developer  | true  | false
       :maintainer | false | false
-      :maintainer | true  | false
+      :maintainer | true  | true
       :owner      | false | false
       :owner      | true  | true
       :admin      | true  | true