Update Ruby to 2.7.4

Closes https://gitlab.com/gitlab-org/gitaly/-/issues/3739

Note that we do not have to touch CI definitions because with gitlab-build-images!428 (merged) we will automatically download the most current 2.7.

We should probably wait for gitlab-development-kit!2137 (merged) to land so developers have a working 2.7.4 setup.