Test against Ruby 3.0 and 3.1

[DannyBen]

I believe Ruby 3.2 should also be added, but not sure if RVM has it yet.
Also, perhaps consider merging any of the PRs that switch from Travis to GitHub Actions? #726 / #725

[jeppeliisberg]

👍 for 3.2

       google-protobuf-3.21.12-x86_64-linux requires ruby version < 3.2.dev, >= 2.5,
       which is incompatible with the current version, 3.2.0

[rmm5t]

I think the path forward would be to move from Travis-CI to GitHub Actions. i.e. See #725 and/or #726