feat(config/users): add support for debian-11 [skip ci]

Avoid this failure when installing the package:

```
Setting up prometheus (2.24.1+ds-1+b5) ...
adduser: The user `prometheus' already exists, but is not a system user. Exiting.
dpkg: error processing package prometheus (--configure):
 installed prometheus package post-installation script subprocess returned error exit status 1
Errors were encountered while processing:
 prometheus
```

prometheus/config/users.sls

@@ -21,6 +21,9 @@ prometheus-config-users-install-{{ name }}-user-present:
     - shell: /bin/false
                   {%- if grains.kernel|lower == 'linux' %}
     - createhome: false
+                    {%- if grains.get('osfinger', '') in ['Debian-11'] %}
+    - system: true
+                    {%- endif %}
                   {%- elif grains.os_family == 'MacOS' %}
     - unless: /usr/bin/dscl . list /Users | grep {{ name }} >/dev/null 2>&1
                   {%- endif %}