chore: bump knative version to 0.18 in update-deps.sh

The newest knative/pkg release has released some change to how metrics
are generated. In particular, the new code stops generating a metrics
for every resource that is reconciled. This dramatically reduces the
metrics cardinality for the webhook. This ultimately
reduces amount of memory required by the webhook daemons and
greatly improves the usability of the /metrics endpoint as it will not
longer return a metrics for every resource that is reconciled.

fixes: tektoncd#2872

hack/update-deps.sh

@@ -22,7 +22,7 @@ source $(git rev-parse --show-toplevel)/vendor/github.com/tektoncd/plumbing/scri
 
 cd ${REPO_ROOT_DIR}
 
-VERSION="release-0.17"
+VERSION="release-0.18"
 
 # The list of dependencies that we track at HEAD and periodically
 # float forward in this repository.