Change S3 to object storage in migration doc.
Replace S3 with object storage as per https://gitlab.com/charts/gitlab/merge_requests/507#note_108204710
The 17.0 major release is coming on May 16, 2024! This version brings many exciting improvements to GitLab, but also removes some deprecated features. We are introducing three breaking change windows during which we expect breaking changes to be deployed to GitLab.com. You can read more about it on our blogpost. The second breaking change window begins 2024-04-29 09:00 UTC and ends 2024-05-01 22:00 UTC.
Replace S3 with object storage as per https://gitlab.com/charts/gitlab/merge_requests/507#note_108204710