Document manaul steps to migrate terraform_states to object storage

Problem

Currently the rake task to migrate terraform_states to object storage is not yet implemented, and the related documentation doesn't have any directions on how to do the migration manually.

Proposal

1. Figure out how to do the migration manually.
2. Document the results.

From a slack discussion:

@tigerwnz depending on the GitLab version the state may be stored in either the terraform_states table, or the terraform_state_versions table (as of 13.5 every push to a state creates a new version record so the history is preserved).

Initial steps to try:

• Copy the files manually to the external bucket, for example using azcopy (azure copy).
• Update some table(s).
  • Experiment with updating terraform_states table, changing the file_store column value for all rows from 1 (local) to 2 remote.