Update istio/ztunnel Docker tag to v1.23.6

This MR contains the following updates:

Package	Type	Update	Change
istio/ztunnel		patch	1.23.5 -> 1.23.6
istio/ztunnel	ironbank-docker	patch	1.23.5-distroless -> 1.23.6-distroless
istio/ztunnel	stage	patch	1.23.5-distroless -> 1.23.6-distroless

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