Use CMake's protobuf module

[Ryanf55]

🎉 New feature

Relates to https://github.com/gazebosim/gz-cmake/issues/9

Summary

This is removing the need for the custom find script for protobuf.

Test it

Compile on CI

Checklist

• [ ] Signed all commits for DCO
• [ ] Added tests
• [ ] Added example and/or tutorial
• [ ] Updated documentation (as needed)
• [ ] Updated migration guide (as needed)
• [ ] Consider updating Python bindings (if the library has them)
• [ ] codecheck passed (See contributing)
• [ ] All tests passed (See test coverage)
• [ ] While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[Ryanf55]

Closing for now to reduce the noise. It can be revisited when CMake+Protobuf work out of the box nicely again.