chore(dns): remove DNS_SERVER, DNS_KEEP_NAMESERVER and replace DNS_ADDRESS with DNS_UPSTREAM_PLAIN_ADDRESSES (#2988)

- Remove `DNS_SERVER` (aka DOT) option: the DNS server forwarder part is now always enabled (see below why)
- Remove `DNS_KEEP_NAMESERVER`: the container will always use the built-in DNS server forwarder, because it can handle now local names with local resolvers (see #2970), it can use the `plain` upstream type (see https://github.com/qdm12/gluetun/commit/5ed6e8292278b54bb5081de0e8ccd0d63a275b3c) AND you can use `DNS_UPSTREAM_PLAIN_ADDRESSES` (see below)
- Replace `DNS_ADDRESS` with `DNS_UPSTREAM_PLAIN_ADDRESSES`:
  - New CSV format with port, for example `ip1:port1,ip2:port2`
  - requires `DNS_UPSTREAM_TYPE=plain` to be set to use `DNS_UPSTREAM_PLAIN_ADDRESSES` (unless using retro `DNS_ADDRESS`)
  - retrocompatibility with `DNS_ADDRESS`. If set, force upstream type to plain and empty user-picked providers. 127.0.0.1 is now ignored since it's always set to this value internally.
  - Warning log on using private upstream resolvers updated
- Warning log if using a private IP address for the plain DNS server which is not in your local subnets
All in all, this greatly simplifies code and available options (less options for the same features is a win). It also allows you to specify multiple plain DNS resolvers on ports other than 53 if needed.

Dockerfile

@@ -175,7 +175,6 @@ ENV VPN_SERVICE_PROVIDER=pia \
     HEALTH_SMALL_CHECK_TYPE=icmp \
     HEALTH_RESTART_VPN=on \
     # DNS
-    DNS_SERVER=on \
     DNS_UPSTREAM_RESOLVER_TYPE=DoT \
     DNS_UPSTREAM_RESOLVERS=cloudflare \
     DNS_BLOCK_IPS= \
@@ -188,8 +187,7 @@ ENV VPN_SERVICE_PROVIDER=pia \
     DNS_UNBLOCK_HOSTNAMES= \
     DNS_REBINDING_PROTECTION_EXEMPT_HOSTNAMES= \
     DNS_UPDATE_PERIOD=24h \
-    DNS_ADDRESS=127.0.0.1 \
-    DNS_KEEP_NAMESERVER=off \
+    DNS_UPSTREAM_PLAIN_ADDRESSES= \
     # HTTP proxy
     HTTPPROXY= \
     HTTPPROXY_LOG=off \