dns: increase TimeToVisible before DNS unavailable warning (#13317)

Updates tailscale/tailscale#13314

Some users are reporting 'DNS unavailable' spurious (?) warnings, especially on Android:

https://old.reddit.com/r/Tailscale/comments/1f2ow3w/health_warning_dns_unavailable_on_tailscale/
https://old.reddit.com/r/Tailscale/comments/1f3l2il/health_warnings_dns_unavailable_what_does_it_mean/

I suspect this is caused by having a too low TimeToVisible setting on the Warnable, which triggers the unhealthy state during slow network transitions.

Signed-off-by: Andrea Gottardo <andrea@gottardo.me>

net/dns/resolver/forwarder.go

@@ -181,7 +181,7 @@ var dnsForwarderFailing = health.Register(&health.Warnable{
 	DependsOn:           []*health.Warnable{health.NetworkStatusWarnable},
 	Text:                health.StaticMessage("Tailscale can't reach the configured DNS servers. Internet connectivity may be affected."),
 	ImpactsConnectivity: true,
-	TimeToVisible:       5 * time.Second,
+	TimeToVisible:       15 * time.Second,
 })
 
 type route struct {