Add back Rake tasks to allow Omnibus GitLab upgrades to work again

What does this MR do and why?

!133394 (merged) dropped some Rake tasks that are used by Omnibus GitLab in the pre-installation phase, but this caused upgrades to fail. The pre-installation script runs:

gitlab-rake gitlab:storage:legacy_attachments gitlab:storage:legacy_projects

If these Rake tasks aren't present, then this check will fail, blocking the upgrade.

To avoid this failure, add back these two Rake tasks with print statements that show no unmigrated projects or attachments. We can drop these calls in Omnibus GitLab in GitLab 16.6 or later.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.