AdguardHome: Add switch from DNS-over-HTTPS to IPv4 (#511)

Author: Antiz96

Services/AdguardHome.md

@@ -28,6 +28,7 @@ Once first setup is done, you can access the Web interface on this URL:
 
 Configuration can be done via the WebUI or directly on the server by editing the `/var/lib/adguardhome/AdGuardHome.yaml` file (requires a restart of the service for changes to be applied).
 
+- I had to switch the upstream DNS server from DNS-over-HTTPS (`https://dns10.quad9.net/dns-query`) to IPv4 (`9.9.9.9`), otherwise I was suffering from random DNS resolution failures due to failed queries / connection closing prematurely (I also added `1.1.1.1` as a secondary upstream DNS server).
 - I had to set the `Rate limit` parameter to `0` (otherwise DNS requests were rate limited, most likely because of Zabbix).
 - I had to disable the resolving of IPv6 addresses by checking the related parameter under the DNS settings in order to avoid unexpected NXDOMAIN responses (e.g. with `Homepage`).
 - I had to disable `system-resolved` and switch back to a static entry in `/etc/resolv.conf` to avoid Adguard's service to fail due to conflicting binding between `systemd-resolved`'s stub listener and Adguard (which both want to bind port 53).