Update quay.io/brancz/kube-rbac-proxy Docker tag to v0.18.1

This MR contains the following updates:

Package	Type	Update	Change
quay.io/brancz/kube-rbac-proxy		patch	v0.18.0 -> v0.18.1
quay.io/brancz/kube-rbac-proxy	ironbank-docker	patch	v0.18.0 -> v0.18.1
quay.io/brancz/kube-rbac-proxy	stage	patch	v0.18.0 -> v0.18.1

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