Hi
Direct upgrade from 7.1 to 8.0 will fail in db migrate steps. Even I faced the similar problem.Instead please take two steps to upgrade from 7.1 to 8.0.
Gitlab 7.1 to Gitlab 7.4
Gitlab 7.4 to Gitlab 7.6
Gitlab 7.6 to Gitlab 8.0
I have documented my experiences doing a similar upgrade you can have a look. Please post questions if any.