OWNERS: remove serathius, add mengjiao-liu, promote pohly

Serathius is focused on etcd these days, we shouldn't rely on him anymore for
approvals. Mengjiao has been very active with logging and would be a good
addition to the pool of reviewers.

I myself (pohly) have been the de-facto maintainer. Being able to approve
changes from others will be useful and I'll be careful to get a
second approver for my own changes.

OWNERS

@@ -1,14 +1,16 @@
 # See the OWNERS docs at https://go.k8s.io/owners
 reviewers:
   - harshanarayana
+  - mengjiao-liu
   - pohly
 approvers:
   - dims
+  - pohly
   - thockin
-  - serathius
 emeritus_approvers:
   - brancz
   - justinsb
   - lavalamp
   - piosz
+  - serathius
   - tallclair