Software compatibility

We are running Geocortex Essentials 4.7.0.566 and Viewer for HTML5 2.8.0 with ArcGIS Enterprise and Server 10.3. We would like to upgrade to Geocortex Essentials 4.12 and ArcGIS Enterprise and Server 10.7. What is the best sequence for this? Should we upgrade ArcGIS first or Geocortex? Do these need to happen at the same time? From the Release Notes 4.7.0 has been tested on ArcGIS 10.5 - does that mean that it won't work with 10.7?