health/dns: reduce severity of DNS unavailable warning (#13152)

`DNS unavailable` was marked as a high severity warning. On Android (and other platforms), these trigger a system notification. Here we reduce the severity level to medium. A medium severity warning will still display the warning icon on platforms with a tray icon because of the `ImpactsConnectivity=true` flag being set here, but it won't show a notification anymore. If people enter an area with bad cellular reception, they're bound to receive so many of these notifications and we need to reduce notification fatigue.

Signed-off-by: Andrea Gottardo <andrea@tailscale.com>

net/dns/resolver/forwarder.go

@@ -177,7 +177,7 @@ func clampEDNSSize(packet []byte, maxSize uint16) {
 var dnsForwarderFailing = health.Register(&health.Warnable{
 	Code:                "dns-forward-failing",
 	Title:               "DNS unavailable",
-	Severity:            health.SeverityHigh,
+	Severity:            health.SeverityMedium,
 	DependsOn:           []*health.Warnable{health.NetworkStatusWarnable},
 	Text:                health.StaticMessage("Tailscale can't reach the configured DNS servers. Internet connectivity may be affected."),
 	ImpactsConnectivity: true,