docs: Update the enforcement date for free user limits

What does this MR do and why?

Updates the enforcement date from 2022-06-22 to 2022-09-15 for user limits.

We should not include GitLab version or milestone since it is not related to the enforcement process on the SaaS production.

See https://about.gitlab.com/blog/2022/03/24/efficient-free-tier/

• Upstream MR: gitlab-com/www-gitlab-com!105352 (merged)