Protocol environment: Update Zarith to 1.12

Context

This MR fixes the constraint we had on Zarith to allow newer versions. The changes I made were what I hit specifically but we could probably change all external -> val to be more future proof.

Manually testing the MR

If we can install and run tests with these changes that should be enough to test it.

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, the Development Version section of CHANGES.md for everything else).
• Select suitable reviewers using the Reviewers field below.