If you check my geojson link, you can notice that I already enabled CORS and added kbn-version.
Doing a test
GET against your server providing GeoJSON, I'm not seeing anything like the following in the response-headers:
That second link that @rashmi provided, which you also commented on provides a link on how to configure this setting on one server, but the specifics of how to do it are likely different for your server. With regard to your comment there on reopening the issue, let's hold off that until we've firmly established there's a common issue that others are likely to experience.
For your response headers, if you believe differently, could you please provide a screenshot of what you're seeing?
Some of the other guidance in that issue is also still good. Is there any chance you can update to > 6.5? Even better, if you can use a version newer than 6.7, you'd get access to the new Maps App which will be replacing the map product you're using now in the long run and receives more frequent improvements and updates.