Update grafana/grafana-image-renderer Docker tag to v3.11.6

This MR contains the following updates:

Package	Type	Update	Change
grafana/grafana-image-renderer		patch	3.11.5 -> 3.11.6
grafana/grafana-image-renderer	ironbank-docker	patch	3.11.5 -> 3.11.6
grafana/grafana-image-renderer	stage	patch	3.11.5 -> 3.11.6

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