However, running the update shows:
root@fsgit:~# sudo apt-get update && sudo apt-get install gitlab-ce
…
gitlab-ce is already the newest version (11.1.0-ce.0).
It seems like you don’t have an apt source configured that carries the newer versions of GitLab. Look at the installation guide, download the script mentioned there and read it to figure out what to add to your sources.list.
Addition: Newest version is 12.4.2, so you’re far from the newest version.
