fix(ci): Pin protobuf

[merkata]

Overview

Pin protobuf to a sensible version that makes juju>=3.0 and py-macaroon-bakery happy, so we don't end up with failing integration tests due to not being able to resolve the dependency version of protobuf.

Rationale

Make CI happy.

Juju Events Changes

n/a

Module Changes

n/a

Library Changes

n/a

Checklist

• [x ] The charm style guide was applied
• [x ] The contributing guide was applied
• [x ] The changes are compliant with ISD054 - Manging Charm Complexity
• [x ] The documentation is generated using src-docs
• [x ] The documentation for charmhub is updated.
• [x ] The PR is tagged with appropriate label (urgent, trivial, complex)