Move to Go protobuf API v2

[adriansmares]

Summary

Closes #2798
References https://github.com/TheThingsIndustries/lorawan-stack/pull/3445

Changes

• Use API v2 protobuf inside our code base. The enterprise PR contains the reviews and more comments at large.

Testing

Enterprise tested with unit tests, end to end tests and staging deployment.

Regressions

N/A.

Notes for Reviewers

This is just CI. No need to review.

Checklist

• Scope: The referenced issue is addressed, there are no unrelated changes.
• Compatibility: The changes are backwards compatible with existing API, storage, configuration and CLI, according to the compatibility commitments in README.md for the chosen target branch.
• Documentation: Relevant documentation is added or updated.
• Changelog: Significant features, behavior changes, deprecations and fixes are added to CHANGELOG.md.
• Commits: Commit messages follow guidelines in CONTRIBUTING.md, there are no fixup commits left.