Debian version shown is incorrect

The Graylog, OS and kernel version shown in the bottom of every web interface page shows an incorrect Debian version.

## Expected Behavior

The text should show Debian version 10.11 on Debian 10.11.

## Current Behavior

The text shows Debian 11.0.12, which is incorrect. Also, if I'm not mistaken, Debian releases don't have a patch level in their version number, i.e. the format is always MAJOR.MINOR.

## Possible Solution

The installed Debian version can be obtained from /etc/debian_version.

## Steps to Reproduce (for bugs)

1. Install Graylog on Debian 10.11.
2. Notice that the web interface shows that Graylog is installed on Debian 11.0.12.

## Your Environment

* Graylog Version: 4.2.1
* Java Version: OpenJDK 11.0.12+7-2~deb10u1
* Elasticsearch Version: 7.10.2
* MongoDB Version: 4.4.10
* Operating System: Debian 10.11.
* Browser version: Firefox 78.15.0esr-1~deb11u1