Master: Update gcr.io/tekton-releases/github.com/tektoncd/dashboard/cmd/dashboard Docker tag to v0.18.0

This MR contains the following updates:

Package	Type	Update	Change
gcr.io/tekton-releases/github.com/tektoncd/dashboard/cmd/dashboard	ironbank-docker	minor	v0.17.0 -> v0.18.0
gcr.io/tekton-releases/github.com/tektoncd/dashboard/cmd/dashboard		minor	v0.17.0 -> v0.18.0
gcr.io/tekton-releases/github.com/tektoncd/dashboard/cmd/dashboard	stage	minor	v0.17.0 -> v0.18.0

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