Add TODOs for unversioned grpc and protobuf

[carlocab]

• Have you followed the guidelines for contributing?
• Have you ensured that your commits follow the commit style guide?
• Have you checked that there aren't other open pull requests for the same formula update/change?
• Have you built your formula locally with brew install --build-from-source <formula>, where <formula> is the name of the formula you're submitting?
• Is your test running fine brew test <formula>, where <formula> is the name of the formula you're submitting?
• Does your build pass brew audit --strict <formula> (after doing brew install --build-from-source <formula>)? If this is a new formula, does it pass brew audit --new <formula>?

---

Follow-up to #131405.

• apache-arrow: add TODO for unversioned grpc and protobuf
• etcd-cpp-apiv3: add TODO for unversioned grpc and protobuf
• mavsdk: add TODO for unversioned grpc and protobuf