Update istio/istioctl Docker tag to v1.5.6

This MR contains the following updates:

Package	Type	Update	Change
istio/istioctl	ironbank-docker	patch	1.5.4 -> 1.5.6
istio/istioctl		patch	1.5.4 -> 1.5.6
istio/istioctl	stage	patch	1.5.4 -> 1.5.6

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