Use latest available Ruby 2.7 images

As of gitlab-build-images!428 (merged), we do not match on patch level Ruby versions anymore (though we maintain the 2.7.2 images for now for back-compat reasons.)

The truth is that this never really was the case anyway; asking for 2.7.2 might have delivered a different version, depending on what RUBY_VERSION was at the time.

Going forward, we will only define and ask for images based on Ruby minor versions (2.7, 3.0, etc), which simplifies and accelerates delivery of important patch releases into CI.

With this change, GCK will start using Ruby 2.7.4, or whatever is the latest RUBY_VERSION in the 2.7 line as centrally specified in gitlab-build-images.