Mark client exec proxy as withdrawn

There have been no updates in 1.5 years nor have there been any community requests.  That combined with a high complexity implementation lead me to believe that we will not pursue this KEP.

Author: enj

keps/sig-auth/2718-20210511-client-exec-proxy/kep.yaml

@@ -6,7 +6,7 @@ owning-sig: sig-auth
 participating-sigs:
   - sig-cli
   - sig-api-machinery
-status: implementable
+status: withdrawn
 creation-date: 2021-05-11
 reviewers:
   - "@mikedanese"