Remove obsolete storage upgrade

This MR aims to remove the support of upgrading from a storage version 3.0 to 3.1.

Indeed, 3.0 was introduced in v16 and used until v20. Version 3.1 was introduced in v21. As everyone have switch to v22 since a while (as Rio protocol was activated), we can safely remove the support of upgrading from v20 to v23 (as this MR will land in v23 release).

Manually testing the MR

Not much to do actually

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR