SCORU,Node: Handle the Protocol_migration internal message

Context

Follow-up of !7729 (merged), fixes #4918 (closed).

We deal with the injection of Protocol_migration in the first block of each protocol, starting from N.

Still no test, because the rollup node does not handle very well the protocol upgrades as of now.

Manually testing the MR

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