bump knative

yays:

1.  > _Dropped the startup probe on the queue-proxy which makes the pods
    > start ~500ms quicker on average. (#11965, @markusthoemmes)_

2.  > User-supplied readinessProbes with a probePeriod set greater than zero
    > are no longer silently ignored after pod startup. (#11190, @julz)

see https://github.com/knative/serving/releases/tag/v0.26.0

Signed-off-by: Ciro S. Costa <ciroscosta@vmware.com>

hack/setup.sh

@@ -32,7 +32,7 @@ readonly KUBERNETES_CONTAINER_NAME=cartographer-control-plane
 
 readonly CERT_MANAGER_VERSION=1.5.3
 readonly KAPP_CONTROLLER_VERSION=0.29.0
-readonly KNATIVE_SERVING_VERSION=0.25.0
+readonly KNATIVE_SERVING_VERSION=0.26.0
 readonly KPACK_VERSION=0.4.1
 readonly SECRETGEN_CONTROLLER_VERSION=0.5.0
 readonly SOURCE_CONTROLLER_VERSION=0.17.0