Hi there,
GitLab 8.x is no longer in development, as the last release was in March of 2017.
Staying up to date with the latest releases is always recommended, especially if you are encountering version incompatibility issues.
What issues are you running into with the upgrade path? The documentation on maintenance lays out a good path for you to follow, but it is an update or two behind. Since GitLab recommends that you first upgrade to the latest available minor version within your major version, your upgrade path should look like this:
8.15 -> 8.17.8 -> 9.5.10 -> 10.8.7 -> 11.3.4
Here is an example command for updating an Omnibus install, with a specific release specified.
sudo apt-get install gitlab-ce=8.17.7-ce.0