As per this link:
Your upgrade path is this: Upgrading GitLab | GitLab
11.11.8 -> 12.0.12 -> 12.1.17 -> 12.10.14 -> 13.0.14 -> 13.1.11 -> 13.8.8 -> 13.12.15 -> latest 14.0.Z -> latest 14.1.Z -> latest 14.Y.Z
the command to do that is this:
apt-get install gitlab-ce=11.11.8-ce.0
for example. You can get the exact version name from the package listing, so here for example: gitlab/gitlab-ce - Results for 'gitlab-ce_11.11.8' in gitlab/gitlab-ce
that is filtered for the 11.11.8 version. Mostly it should always finish with -ce.0 but might vary (I kind of remember writing once or twice -ce.1 but I could be wrong).
Also, once that apt upgrade command has finished, you can check background migrations either in the web interface, or run the following commands from this link: Upgrading GitLab | GitLab
gitlab-rails runner -e production 'puts Gitlab::BackgroundMigration.remaining'
gitlab-rails runner -e production 'puts Gitlab::BackgroundMigration.pending'
use sudo on all of those commands if you aren’t doing them as the root user. Once background migrations has completed (showing zero), you can then do the next upgrade so the apt-get upgrade command with the specific version number from the next gitlab version on the upgrade path.