v-m: Redo Internet/DNS connectivity checks (again)

Using domain names in the "blackbox" probe makes it difficult to tell
the difference between a complete Internet outage and DNS issues.  I
switched to using these names when I changed how the firewall routed
traffic to the public DNS servers, since those were the IP addresses
I was using to determine if the Internet was "up."  I think it makes
sense, though, to just ping the upstream gateway for that check.  If
EverFast changes their routing or numbering, we'll just have to update
our checks to match.

victoria-metrics/blackbox.yml

@@ -10,7 +10,7 @@ modules:
     timeout: 2s
   dns_recursive:
     dns:
-      query_name: news.ycombinator.com
+      query_name: github.com
       query_type: A
     prober: dns
     timeout: 5s

victoria-metrics/scrape.yml

@@ -34,10 +34,7 @@ scrape_configs:
     - icmp
   static_configs:
   - targets:
-    - github.com
-    - cloudflare.com
-    - amazonaws.com
-    - azure.com
+    - 23.29.47.1
   relabel_configs:
   - source_labels: [__address__]
     target_label: __param_target