Update istio/install-cni Docker tag to v1.21.4

This MR contains the following updates:

Package	Type	Update	Change
istio/install-cni		patch	1.21.3 -> 1.21.4
istio/install-cni	ironbank-docker	patch	1.21.3-distroless -> 1.21.4-distroless
istio/install-cni	stage	patch	1.21.3-distroless -> 1.21.4-distroless

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