Treat unreachable resolvers as unknown, not warnings

An unreachable resolver tells us nothing about propagation: it's neither
a disagreement nor an error in the zone. Surface it as a distinct
Unreachable state so it stops polluting warning counts and the
Disagreeing/Errored buckets.

checker/rules_resolvers.go

@@ -71,7 +71,7 @@ func (r *resolverLatencyRule) Evaluate(ctx context.Context, obs sdk.ObservationG
 	var states []sdk.CheckState
 	for _, rv := range data.Resolvers {
 		if !rv.Reachable {
-			states = append(states, warnState(CodeResolverUnreachable, rv.ID,
+			states = append(states, unknownState(CodeResolverUnreachable, rv.ID,
 				fmt.Sprintf("resolver %s (%s, %s) did not answer any query", rv.Name, rv.IP, rv.Transport)))
 			continue
 		}