What you have done until the last step is simply a new SonarQube 8.6 installation.
Only the last step (connecting to your 7.9 database) is really the upgrade in itself .
And for the upgrade to complete you must perform a few steps detailed in upgrade documentation
So the screen you see is just normal, SonarQube is waiting for you to finish the upgrade steps.
You should be able to do that without much trouble if you read the doc.
All the database content is preserved. That’s precisely the purpose of the upgrade process: Preserve the historical data from older versions and add/convert some tables to allow the new features.
However our upgrade procedure clearly specify that you must do a DB backup before hitting the Upgrade button in case something goes wrong.