TPA-RFC-14: Reduce default CI artifact retention period to control growth
We recently created a 250 GB volume for GitLab artifacts, as its already almost full (91% at the time of writing).
I think a significant contributing factor is that most projects do not define an artifact retention period, and so the instance default of 30 days is used. We should revisit this value considering the massive uptake GitLab CI has seen in the last weeks, notably for website and Debian package builds.
I'd like to propose that we reduce this period to 10 days instance wide, after a short announcement to stakeholders. Projects that require a longer retention should override it within their project's CI config. Modification of this setting will only apply to new pipelines, so we may still need to expire some artifacts manually.
The latest artifacts for all pipelines are kept indefinitely regardless of this setting, so it only affects artifacts of pipelines which have been superseeded by new ones.