SERVFAIL response does not count as a failure

### Prerequisites

- [X] I have checked the [Wiki](https://github.com/AdguardTeam/AdGuardHome/wiki) and [Discussions](https://github.com/AdguardTeam/AdGuardHome/discussions/categories/q-a) and found no answer

- [X] I have searched other issues and found no duplicates

- [X] I want to report a bug and not [ask a question or ask for help](https://github.com/AdguardTeam/AdGuardHome/discussions/categories/q-a)

- [X] I have set up AdGuard Home correctly and [configured clients to use it](https://github.com/AdguardTeam/AdGuardHome/wiki/Clients). (Use the [Discussions](https://github.com/AdguardTeam/AdGuardHome/discussions/categories/q-a) for help with installing and configuring clients.)

### Platform (OS and CPU architecture)

Linux, AMD64 (aka x86_64)

### Installation

Docker

### Setup

On a router, DHCP is handled by the router

### AdGuard Home version

v0.107.52

### Action

This evening, cloudflare's DNS was having issues, as reported on their Status Page.
When doing an NSLOOKUP, the server would respond with SERVFAIL


My adguardhome instance uses 1 upstream DNS providers:
````
tls://.cloudflare-gateway.com
````

I have et the fallback DNS server to:
````
quic://unfiltered.adguard-dns.com
````

Adguardhome didn't use the fallback DNS, and instead, my entire network was down due to not being able to resolve names.

In order to temporarily fix the issue, i added ````quic://unfiltered.adguard-dns.com```` as a second DNS server on the list, and set it to load balance. This partially solved the issue, but since AGH didn't consider the SERVFAIL as a failure, some of my requests continued to be sent to cloudflare, and my network had intermitent name resolution.

The only way to fix the issue was to commnent out the cloudflare dns address from my list of upstream resolvers, until they fixed the issue.

### Expected result

AGH should identify SERVFAIL responses as being a bad response, and use the fallback resolver.

In load balanced mode, AGH should identify a SERVFAIL response and stop using that upstream server for a while.

### Actual result

AGH continued to act as if cloudflare was giving good DNS responses, despite the SERVFAIL status.


### Additional information and/or screenshots

_No response_