Update `ruby:2.7` usages in spec/
We have a bunch of ruby:2.7
occurrences left unchanged in our Ruby 3 branch.
Most of them in specs (85). From what I've seen, these are just string constants used for examples (as values) and should not affect the build itself.
To finish the transition to Ruby 3, let's update them as well where needed.
I don't think we should add all the changes into !70798 (closed), because it will bloat the MR.
I suggest merging such changes directly to the upstream, where possible.
Edited by Aleksei Lipniagov