chore(monitoring): stop scraping offline hosts, mute noisy alert levels

- Add an offlineHosts list to VictoriaMetrics scrape generation: SBCs
  and the whole k3s-prod-1 cluster are powered off, shoukei's exporter
  is disabled on the machine; also comment out the dnsmasq-exporter
  job (runs on suzi).
- Disable node-exporter on shoukei: a laptop on untrusted networks
  should not expose :9100 (the firewall is off repo-wide).
- Route severity none|info alerts to the null receiver; meta alerts
  and info noise should never page.

Signed-off-by: Ryan Yin <xiaoyin_c@qq.com>

hosts/12kingdoms-shoukei/default.nix

@@ -21,6 +21,11 @@ in
   services.sunshine.enable = lib.mkForce false;
   services.tuned.ppdSettings.main.default = lib.mkForce "power-saver";
 
+  # This laptop joins untrusted networks and the firewall is disabled
+  # (modules/nixos/base/ssh.nix), so a 0.0.0.0:9100 node-exporter would be exposed
+  # to whatever network it is on. It is no longer scraped anyway - disable it.
+  services.prometheus.exporters.node.enable = lib.mkForce false;
+
   networking = {
     inherit hostName;
     inherit (myvars.networking) nameservers;