Enable CI job archival period on gitlab.com

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Context

After rolling out the fix from Extend job archival mechanism to the whole pipe... (#271614 - closed), we should enable the archive_builds_older_than on Gitlab.com and communicate it clearly to the customers.

Idea: We could start with a 6 months period and then reduce it to 3 months. After 3 months, all jobs in a pipeline are considered archived/read-only.

Previous attempt

gitlab-com/gl-infra/production-engineering#5588 (closed)

Edited by 🤖 GitLab Bot 🤖