fix(dataplane): properly default deprecated service account name

[pmalek]

What this PR does / why we need it:

API server fills in the deprecated service account field based on what's set in the service account name field. This causes the operator to incorrectly assume there's a diff which needs an update on the Deployment.

v1.PodTemplateSpec{
    ObjectMeta: {Labels: {\"app\": \"dataplane-example\", \"dataplane-pod-label\": \"example\", \"gateway-operator.konghq.com/selector\": \"86e60937-f0b6-4816-845f-ece9f4aa6a2a\"}, Annotations: {\"dataplane-pod-annotation\": \"example\"}},
    Spec: v1.PodSpec{
      ... // 8 identical fields
      NodeSelector:                 nil,
      ServiceAccountName:           \"aaa\",
-     DeprecatedServiceAccount:     \"aaa\",
+     DeprecatedServiceAccount:     \"\",
      AutomountServiceAccountToken: nil,
      NodeName:                     \"\",
      ... // 26 identical fields
    },
  }
"}

This PR fixes that by copying what's in serviceAccountName to serviceAccount (deprecated) field.

This is a temporary workaround until #500 (or similar) gets implemented.