Remove CI builds/jobs older than X years

Problem to Solve

Allow users to configure removal of older builds/jobs for their namespace or instance.

Questions to Answer

1. What would it look like to remove CI builds (and all its artifacts) older than X years?
2. How could we enable this on gitlab.com? What comms would we need for our users? Would this be a breaking change, and therefore cannot enable til x.0?
3. What if this could be configurable for Self-Managed and Dedicated customers who manage their own data?

Implementation

See linked MRs 🎉

Follow-up Tasks

• Ensure the setting can only be viewable by those who have the Owner role of a project
• Assign Delete old pipelines (!171142 - merged) for UX Review
• Create a feature flag rollout issue (can we roll out to GitLab team members first?)
• Merge Add docs for automatic pipeline cleanup (!174963 - merged)
• Add a ❓ to the Settings and a link to the docs once ready
• Prepare a 17.7 Release Post linking to automatically delete old pipelines (#338480) - also confirm if this issue is a dupe of that one