Update gcr.io/kasten-images/metric-sidecar Docker tag to v6.5.8

This MR contains the following updates:

Package	Type	Update	Change
gcr.io/kasten-images/metric-sidecar		patch	6.5.5 -> 6.5.8
gcr.io/kasten-images/metric-sidecar	ironbank-docker	patch	6.5.5 -> 6.5.8
gcr.io/kasten-images/metric-sidecar	stage	patch	6.5.5 -> 6.5.8

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