chore: remove defaultMode like upstream did

- per kubernetes-sigs/cluster-api@95e9d0d

Signed-off-by: Chris Privitere <23177737+cprivitere@users.noreply.github.com>

Author: cprivitere

config/default/manager_webhook_patch.yaml

@@ -19,5 +19,4 @@ spec:
       volumes:
       - name: cert
         secret:
-          defaultMode: 420
           secretName: $(SERVICE_NAME)-cert