Migration from v12 Backup to v16

[adityaduggal]

Issue: When we restore the v12 backup of rigb DB then we are unable to migrate to v16 bench
Steps:

1. Install frappe v16 and erpnext v16
2. Restore backup of v12 rigb or rigpl DB
3. bench --site v16rigb migrate fails

Since the migration is important to test the details of the data integrity for production migration we need to this on both the DB and apps.
Need to have a detailed doc or steps for both the apps.