Upgrade to ruby 2.7.4

Ruby 2.7.4 has been released.
https://www.ruby-lang.org/en/news/2021/07/07/ruby-2-7-4-released/

This includes security updates for issues that have been causing Critical / High findings in customer scans of Gitlab.

We should consider updating to this most recent patch.

Link to most recent ruby update:

• &2380 (closed)

Required MRs

• gitlab-rails: !68363 (merged)
• gitaly-ruby: gitaly!3771 (merged)
• build images: gitlab-build-images!428 (merged)
• CNG: gitlab-org/build/CNG!739 (merged)
• omnibus: omnibus-gitlab!5545 (merged)
• gitlab chart: gitlab-org/charts/gitlab!2162 (merged)
• GDK: gitlab-development-kit!2137 (merged)
• GCK: gitlab-compose-kit!176 (merged)
• labkit: https://gitlab.com/gitlab-org/labkit-ruby/-/merge_requests/79
• gitlab-exporter: https://gitlab.com/gitlab-org/gitlab-exporter/-/merge_requests/150
• gitlab-experiment: https://gitlab.com/gitlab-org/gitlab-experiment/-/merge_requests/128
• gollum: I don't think it's needed, since it has no .ruby-version and asks for 2.7 in CI

Release notes

The version of Ruby used by GitLab has been updated to 2.7.4 in order to mitigate security concerns.