Deprecate support for Prometheus 1.x rule format

[alanprot]

We keep an entire legacy promql engine code on cortex just to support this old rule format and I think is time for it to retire!

This is also only available on the configdb (and not on the most recents backend implementation like s3, azure or gcp)

https://github.com/cortexproject/cortex/blob/362e902a5da3d6a3bd787f2e9e11da44db3c2372/pkg/configs/userconfig/config.go#L248-L257