exec credential provider: latest milestone is v1.21, per comment

Signed-off-by: Andrew Keesler <[email protected]>

Author: ankeesler

keps/sig-auth/541-external-credential-providers/kep.yaml

@@ -18,7 +18,7 @@ approvers:
 prr-approvers:
   - "@deads2k"
 stage: beta
-latest-milestone: "v1.20"
+latest-milestone: "v1.21"
 milestone:
   alpha: "v1.10"
   beta: "v1.11"