Update kubebuilder/kube-rbac-proxy Docker tag to v0.15.0

This MR contains the following updates:

Package	Type	Update	Change
kubebuilder/kube-rbac-proxy	ironbank-docker	minor	v0.14.4 -> v0.15.0
kubebuilder/kube-rbac-proxy	stage	minor	v0.14.4 -> v0.15.0

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