Allows disabling bootstrap_dns completely.

### 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

Currently unable to clear all DNS for bootstrap_dns.
If I delete them all it automatically reverts to the default set of DNS.

### Proposed solution

Allows me to delete the entire DNS of bootstrap_dns. Do not revert to the default DNS list.
This way I can disable bootstrap_dns.

### Alternatives considered and additional information

Fill in a wrong DNS. e.g. 0.0.0.0