Why clean migration is required, How to support index upgrade?

https://github.com/subsquid/squid-subsquid-network/blob/e86b9a6a97b6d842ecb33978be328637e92b342f/commands.json#L34

If each migration deletes the previous one, it will be impossible to upgrade based on the original index.
Maybe you can add a --force option, if enabled, to clean up the migration

