WASM PVM: Restore Nairobi protocol

Context

Removing a variant in the type [Pvm_input_kind.protocol] is forbidden, because
it alters how the WASM PVM behaves to the protocol migration message. Although
this breaking change has no consequence in the real world, we should strive to
keep the WASM PVM backward compatible in any circumstances.

Manually testing the MR

This is a manual revert of f3a797c4. We should check I didn’t forget anything.

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