Prevent upgrade to 14.0 if there is data on legacy storage

What does this MR do?

Prevents upgrade to 14.0 if there is any data on legacy storage. We've deprecated legacy storage in GitLab 13.0 and are now removing it in %14.0. Due to the nature of that change, we need to prevent updating an existing installation to 14.x, if there is still unmigrated data to hashed storage.

Related issues

#5984 (closed)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• Pipeline is green on dev.gitlab.org if the change is touching anything besides documentation or internal cookbooks
• trigger-package has a green pipeline running against latest commit

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for the GitLab Chart opened