Update for replset member ping time and last heartbeat in "MongoDB Replset Summary" dashboard

It looks as though we don't have compatibility mappings for the metrics behind "Max Member Ping Time", "Max Heartbeat Time" graph panels in the MongoDB Replset Summary dashboard, so they're showing empty in PMM 2.11 and 2.12 which uses the new v0.20 mongodb_exporter.

I suggest we update the graph panel's equations to use the new-style metric names.

The prometheus equations that work for me in a quick test are:

mongodb_rs_members_pingMs{service_name=~"$service_name"}

time() - mongodb_members_lastHeartbeat{service_name=~"$service_name"}/1000

This is before any grouping to get max(), but I don't see why we need to only show max for these particular ones. The context is already reduced down to one replicaset in the first place.