Nautobot: Authorization bypass in approval workflow REST API allows self-approval and unauthorized activation of scheduled jobs

### Impact This is an authorization bypass that escalates into unauthorized server-side job execution. 1. Primary impact - self-approval: The approver checks (approver-group membership, `change` permission on the object under review, one-response-per-user) are enforced only in the `approve`/`deny` actions on `ApprovalWorkflowStage`. The generic `ApprovalWorkflowStageResponse` create endpoint enforces none of them, so a user holding only `extras.add_approvalworkflowstageresponse` can `POST` approved responses directly and drive a stage past its `min_approvers` threshold with no legitimate approver involved. The serializer also exposes `user` and `state` as writable, letting the attacker attribute responses to arbitrary users. 2. Escalated impact job activation: Because the approval workflow gates a `ScheduledJob`, self-approval does not stop at the approval record. Stage approval cascades through `ApprovalWorkflow.save()` to `on_workflow_approved()`, which sets the target job to enabled/active. ### Patches Fixed in Nautobot v3.1.8 ### Workarounds Restrict the `extras.add_approvalworkflowstageresponse` permission so that no untrusted user holds it; approval responses should only ever be created via the stage `approve`/`deny` actions. There is no configuration flag that disables the generic create endpoint prior to the patch.