Remove Ruby 2.7 support

What does this MR do and why?

There are build failures related to Ruby 2.7: https://gitlab.com/gitlab-org/ruby/gems/devfile-gem/-/pipelines/1133599653. Related thread: !52 (comment 1734240269)

We no longer need ruby 2.7 support in the monolith, so we can remove support for it from this gem.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.