Disable editing group approval rules in merge requests

Group Approval Rules are never editable whereas Project and Merge Request may be. On the Merge Request page approval rules are supplied to the UI. We need to specify the type of approval rule for each so the front end can determine if they are editable.

On the frontend we will need to block the user from editing GroupApprovalRules in a merge request. On the backend we will need make sure group approval rules are not added to MergeRequest::CreateService approval_rules_attributes.

As discussed in &11451 (comment 1747834662):

we should specifically make group level approval rules uneditable when creating an MR. The approval rule type should be supplied to the frontend so this is clear to users. All other rules should function as they do currently.