contracts: Fix incorrect storage alias in mirgration (#1687)

# Description

We are recently trying to upgrade our Substrate version from
`polkadot-v0.9.43` to `polkadot-v1.0.0` and we noticed a critical issue:
all deployed contracts seem to be experiencing a `CodeNotFound` error.
After a thorough investigation, it appears that the root cause of this
issue lies in the mismatch between the storage alias of `CodeInfoOf<T>`
in the migration and its original definition.

This PR corrects the storage alias to align it with its original
definition

I am uncertain about the proper approach for adding tests to this
change. Would the team consider taking over this PR to bring it to
completion?

---------

Co-authored-by: pgherveou <[email protected]>