upgrade Gitaly to newest version on the third Thursday of each month

Gitaly container images only provide a label matching minor (and not major) releases, which means we need to manually chase those every month. This is a known upstream issue we hope gets fixed upstream. If it doesn't we should probably setup some automation on our end to work around it.

In the meantime, just perform the manual upgrade on a regular basis.

According to GitLab's versioning policy, minor releases are published on the third Thursday of each month. For July, for example, I believe this is today, 2025-07-17, and, magic, 18.2 just hit their blog.

Assigning this myself who will be star during the next occurence, which is august 21st, but also to chase that specific upgrade already.