chore(policy): set an informative field manager on patches

[olix0r]

When the policy controller patches a status, it sets the field manager to be that of the Kind of resource being managed. Per the Kubernetes documentation, this field should describe the controller that is making the change:

Managers identify distinct workflows that are modifying the object (especially
useful on conflicts!), and can be specified through the fieldManager query
parameter as part of a modifying request. When you Apply to a resource, the
fieldManager parameter is required. For other updates, the API server infers a
field manager identity from the "User-Agent:" HTTP header (if present).

When you use the kubectl tool to perform a Server-Side Apply operation,
kubectl sets the manager identity to "kubectl" by default.

This commit sets the field manager to "linkerd.io/policy-controller", as is used in status values.

[olix0r]

Before:

    - apiVersion: gateway.networking.k8s.io/v1beta1
      fieldsType: FieldsV1
      fieldsV1:
        f:status:
          .: {}
          f:parents: {}
      manager: gateway.networking.k8s.io
      operation: Update
      subresource: status
      time: "2024-11-25T18:40:30Z"

After:

    - apiVersion: gateway.networking.k8s.io/v1beta1
      fieldsType: FieldsV1
      fieldsV1:
        f:status:
          f:parents: {}
      manager: linkerd.io/policy-controller
      operation: Update
      subresource: status
      time: "2024-11-25T18:45:06Z"