Explicitly set Omnibus and CNG Ruby version in CI
What does this MR do and why?
This commit adds OMNIBUS_RUBY_VERSION and CNG_RUBY_VERSION to
define which versions are used in the Omnibus and CNG projects.
Previously the Ruby versions used in the child pipelines for building
Omnibus and CNG was determined based on the image used in the
respective CI jobs. When we bumped the CI image to 3.1.5 in
gitlab-build-images!803 (merged),
this quietly added a requirement to Omnibus that Ruby 3.1.5 had to be
supported. At the same time, the build-cng image used Ruby 3.1.4, so
https://gitlab.com/gitlab-org/build/CNG-mirror/-/jobs/6821715942
started to fail once CNG stopped building Ruby 3.1.4.
While the latter case needed gitlab-org/build/CNG!1828 (merged), we can at least avoid the first problem by explicitly setting the version needed. In an ideal world, these variables would be determined automatically from the upstream projects. However, setting these variables avoids surprise failures and opens the possibility up for automation to update these values.
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