Yes, LTS are to 5 years. 16.04 is April 2016, so the five years passed 
Just be careful when doing the upgrade, I’ve had occurrences when using do-release-upgrade that it removed packages that were installed outside of the Ubuntu repos. You might be safe, but you have to make sure in the list of packages being removed that gitlab-ce or gitlab-ee isn’t one of them. You can get around this however by using the standard upgrade methods for Debian, which is basically edit /etc/apt/sources.list to replace xenial with bionic. Then you would just do:
apt-get clean all
apt-get update
apt-get upgrade
apt-get update
apt-get upgrade
apt-get dist-upgrade
yes, on purpose there are a couple of commands repeated, as sometimes after the first apt-get upgrade, I’ve had times I refresh the repository and have to run it again as some other packages became available. After this the dist-upgrade gets you fully upgraded. Then reboot, and you are just left with a small cleanup so:
apt-get autoremove
that should pretty much do it. But try the do-release-upgrade option first as this will probably clean up old packages better since the other method some can get left behind for Xenial which then requires finding them and manually removing them.