Disk space issues and log rotation. (OpenWRT)

### Prerequisites

Please answer the following questions for yourself before submitting an issue. **YOU MAY DELETE THE PREREQUISITES SECTION.**

- [X ] I am running the latest version
- [X ] I checked the documentation and found no answer
- [X ] I checked to make sure that this issue has not already been filed

### Issue Details

* **Version of AdGuard Home server:**
* Updating from v0.107.0-a.198+1e729601 to v0.107.0-a.199+2fc10848.
* **How did you install AdGuard Home:**
* Originally installed via AGH script. Edge build

* **If it's a router or IoT, please write device model:**
* BT Home Hub 5
* **CPU architecture:**
* (MIPS)
* **Operating system and version:**
* ( OpenWrt 21.02.0, r16279-5cc0535800)

### Expected Behavior

Currently AGH copies its binary and config to the /opt/AdGuardHome/agh-backup folder on upgrade. However it does not appear to check for available diskspace before it does so and thus on limited space filesystems AGH fails the upgrade and does not report reason why on the control panel (however it does report on the router logs). A error message back to the web console would be appreciated.

Also AGH recently changed how it managed logs/query logging but there is no way to have it purge the old logs at the rotation leading to double the disk space needed?
https://github.com/AdguardTeam/AdGuardHome/commit/1e72960140cbc5f932650f2202ec6290a70185c0

At present i manage these issues by manually deleting the rotated logs and purging the backed up copy of AGH once it has updated to stop AGH running out of disk space. A method for managing this or even a warning to the console that disk space is low would be a good addition.