Draft: chore(mimir): casually drop any alerts in the 'mimir' env

Second part of : gitlab-com/gl-infra/scalability#2684 (closed)

This drops all alerts in the 'mimir' env or ruler_cluster labelset, which is (as far as I can tell from rules/mimir in runbooks the equivalent to 'thanos-rule')

You might ask why this isnt a route block in AM, and the answer is that its technically more efficient to drop them before they reach AM but also jsonnet is a hell onto itself