Fix extension of GEOJSON link in QgsWfs3DescribeCollectionHandler

[manisandro]

The OAPIF collections/<layername> page contains a link

        {
            "href": "<...>/collections/<layername>/items.json",
            "rel": "items",
            "title": "Edit Lines items as GEOJSON",
            "type": "application/geo+json"
        },

which json as extension even though the content-type is application/geo+json. The collections/<layername>/items.json however contains the self-link as

        {
            "href": "<...>/collections/<layername>/items.geojson",
            "rel": "self",
            "title": "Retrieve the features of the collection as GEOJSON",
            "type": "application/geo+json"
        },

This PR ensures that also the former link uses GEOJSON as extension.

[github-actions]

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[elpaso]

LGTM but you also need to update the test control files.