Modify how the plugin checks supported GeoNode versions

[Gpetrak]

The QGISGeoNode plugin currently uses the version.txt file to determine supported GeoNode versions by accessing the URL:
<geonode_instance_domain>/version.txt.

However, starting with GeoNode 5, this endpoint requires authentication, which prevents the plugin from validating the version successfully.

To address this, we plan to modify the way the plugin checks the GeoNode version. Instead of using the version.txt file, the plugin will verify the GeoNode API client version by checking if the response from /api/v2 is valid.

[giohappy]

We should find the metadata endpoint inside the api/v2 index. At the moment it's not listed becase it's not recorded inside the API router in GeoNode, but it will be fixed: GeoNode/geonode#13694