Remove legacy snapshot import

This MR aims to remove an obsolete snapshot import upgrade.

Indeed, since we have switch to protocol Rio, v7 snapshots cannot be imported anymore. Thus we can remove this upgrade as we cannot export them anyway.

In addition to that, the notion of legacy was a bit too loose and we were actually running the upgrade when importing the v8 snapshots since we have bumped the snapshot's version de v9 (see !18387 (merged)). As a side effect, importing a v8 snapshot was triggering, wrongly, the upgrade, resulting in a failing full snapshot import.

This wrong behavior was not caught by the tezt tests as we have no infrastructure to test legacy snapshots or outdated storages (as it requires to have different octez version available in tezt).

This MR also introduce an "hardcoded" version check to make sure that the v8 legacy upgrade is tied to v8 and not only on the notion of "legacy snasphot".

Manually testing the MR

Download a full snapshot (rionet is the lightest). Try to import it with/without this MR. Do not add the --no-check flag as it helps triggering the is while importing.

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