Update istio/sidecar_injector Docker tag to v1.5.9

This MR contains the following updates:

Package	Type	Update	Change
istio/sidecar_injector	ironbank-docker	patch	1.5.8 -> 1.5.9
istio/sidecar_injector		patch	1.5.8 -> 1.5.9
istio/sidecar_injector	stage	patch	1.5.8 -> 1.5.9

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