Unencrypted DOH Only Mode

### Prerequisites

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

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

- [x] I want to request a feature or enhancement and not ask a question

### The problem

When using unecrypted DOH (via `allow_unencrypted_doh`) and disabling plain DNS + other encrypted DNS mechanisms AdguardHome fails to start with `disabling plain dns requires at least one encrypted protocol`.

### Proposed solution

Allow users to use only unecrypted DOH.

### Alternatives considered and additional information

_No response_