The Server is an older release of OS ? (12.04 in 2016) and GitLab 8.5.8 ) Patching release will solve the concern temporarily and it will not compliant with the latest lease of OS as well as Application.
I have analyzed on up-gradation of this version to Ubuntu 22 and GitLab 15.0.3 Version which takes lots of OS and Application upgrades process and it’s time-consuming, there are suggestions from Gitlab to in-place upgrade is a good choice but it can be dependent and not more reliable.
Considering the above I recommended building the new Server with the latest OS and application and checking the latest code to create a new project, this will ensure all the reliable/valid and Prod code only gets updated on the new server.
Please suggest this so we can have a mode in details discussion.
OS and Application release chart for more details below,
https://gitlab-com.gitlab.io/cs-tools/gitlab-cs-tools/what-is-new-since/?tab=stats