Synology NAS appears to be using an outdated gitlab-ce:v11.x docker image. The docker image is official, but it appears Synology stopped updating/maintaining their bundled offering of it.
As CE is open-source software, third-parties like Synology are free to offer GitLab as a bundled/packaged offering, but maintenance and updates for third-party offerings is the responsibility of the third-party.
All official and suggested ways to install GitLab can be found here:
If you’re able to directly use Docker to install services on the Synology NAS, I believe you should be able to pull and use any version of GitLab you’d like, including the latest 13.3.
If running docker natively on your Synology box isn’t an option, I suggest inquiring with someone at Synology to see what it’d take to update the version on their end.
Let us know if you have any questions or concerns!