Proto: rename first_level_of_Tenderbake for consistency

The migration block is not endorsed by bakers. Therefore it is the "first level of the protocol" that is not endorsed, not the "first level of Tenderbake". This MR simply makes a renaming to make the names consistent with their meaning.

Checklist

• [n/a] Document the interface of any function added or modified (see the coding guidelines)
• [n/a] Document any change to the user interface, including configuration parameters (see node configuration)
• [n/a] Provide automatic testing (see the testing guide).
• [n/a] 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