Make field "Bootstrap DNS servers" accept upstream syntax

### 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

Snapcraft

### Setup

On one machine

### AdGuard Home version

0.107.36

### Action

I attempted to add comments to the "Bootstrap DNS servers" text box. Attempting to apply or test the upstreams results in an error:

```
Error: control/dns_config | checking bootstrap # 9.9.9.10: invalid address: bootstrap # 9.9.9.10:53: ParseAddr("# 9.9.9.10"): unexpected character (at "# 9.9.9.10") | 400
```

Contents that I'm testing with are:

```
# 9.9.9.10
149.112.112.10
2620:fe::10
2620:fe::fe:10
```

### Expected result

The "Examples" section immediately above the textbox indicate that lines leading with `#` are proper syntax for comments, so I would expect the syntax to validate and for AdGuard to ignore the lines indicated as comments

### Actual result

AdGuard fails to validate the config and refuses to apply the settings

### Additional information and/or screenshots

_No response_