upgrade build images to use Go 1.16

Now that Prometheus has [upgraded to Go 1.16](https://github.com/prometheus/prometheus/pull/8544) for version 2.26.0 we should perform a similar upgrade for Cortex.

This will result in the reverted change in RSS memory behavior in Go 1.16. However, it would be good if we could include this in the 1.9.0 Cortex release and observe the effects in the RC.