Update fips-containers.istio.tetratelabs.com/istioctl Docker tag to v1.21.6

This MR contains the following updates:

Package	Type	Update	Change
fips-containers.istio.tetratelabs.com/istioctl		patch	1.21.5 -> 1.21.6
fips-containers.istio.tetratelabs.com/istioctl	ironbank-docker	patch	1.21.5-tetratefips-v0-distroless -> 1.21.6-tetratefips-v0-distroless
fips-containers.istio.tetratelabs.com/istioctl	stage	patch	1.21.5-tetratefips-v0-distroless -> 1.21.6-tetratefips-v0-distroless

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