cluster/kubemark: Remove inactive members from OWNERS

As a part of cleaning up inactive members (those with no activity within
the past 18 months) from OWNERS files, this commit moves gmarek from an
approver to an emeritus_approver.

cluster/kubemark/OWNERS

@@ -1,14 +1,14 @@
 # See the OWNERS docs at https://go.k8s.io/owners
 
 reviewers:
-  - gmarek
   - shyamjvs
   - sig-scalability-reviewers
   - wojtek-t
 approvers:
-  - gmarek
   - shyamjvs
   - sig-scalability-approvers
   - wojtek-t
+emeritus_approvers:
+  - gmarek
 labels:
 - sig/scalability