Update istio/istioctl Docker tag to v1.21.6

This MR contains the following updates:

Package	Type	Update	Change
istio/istioctl		patch	1.21.5 -> 1.21.6
istio/istioctl	ironbank-docker	patch	1.21.5-distroless -> 1.21.6-distroless
istio/istioctl	stage	patch	1.21.5-distroless -> 1.21.6-distroless

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