fix OWNERS file (#5359)

Initially this was to fix a mis-indented approvers key. However, it turns
out that 'oilbeater' is not a member of kubernetes-sigs nor
kubernetes-incubator (the org this repo was migrated from). Thus this
OWNERS file is failing prow's validation check.

As a workaround I've opted to move them to emeritus_approver, which
isn't valiated and can be used as a hint for other approvers in this
repo

roles/network_plugin/kube-ovn/OWNERS

@@ -1,6 +1,4 @@
 # See the OWNERS docs at https://go.k8s.io/owners
 
- approvers:
-  - oilbeater
-reviewers:
+emeritus_approvers:
 - oilbeater