feat(dns): DNS_UPSTREAM_RESOLVER_TYPE option which can be plain or DoT

- Migrate `DOT` to `DNS_SERVER`
- Migrate `DOT_PROVIDERS` to `DNS_UPSTREAM_RESOLVERS`
- Migrate `DOT_PRIVATE_ADDRESS` to `DNS_PRIVATE_ADDRESSES`
- Migrate `DOT_CACHING` to `DNS_CACHING`
- Migrate `DOT_IPV6` to `DNS_UPSTREAM_IPV6`

internal/vpn/tunnelup.go

@@ -46,7 +46,7 @@ func (l *Loop) onTunnelUp(ctx, loopCtx context.Context, data tunnelUpData) {
 		return
 	}
 
-	if *l.dnsLooper.GetSettings().DoTEnabled {
+	if *l.dnsLooper.GetSettings().ServerEnabled {
 		_, _ = l.dnsLooper.ApplyStatus(ctx, constants.Running)
 	} else {
 		err := check.WaitForDNS(ctx, check.Settings{})