Update istio/install-cni Docker tag to v1.15.7

This MR contains the following updates:

Package	Type	Update	Change
istio/install-cni	ironbank-docker	patch	1.15.6 -> 1.15.7
istio/install-cni		patch	1.15.6 -> 1.15.7
istio/install-cni	stage	patch	1.15.6 -> 1.15.7

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