Update to Ruby used for testing, linting to 2.7.4

What does this MR do?

Updates Ruby used in CI (runs rspec and lint jobs) to 2.7.4.

Related issues

Part of gitlab-org/gitlab#335890 (closed)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• Merge Request Title and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for omnibus-gitlab opened
  • Omnibus: gitlab-org/omnibus-gitlab!5545 (merged)
  • CNG: gitlab-org/build/CNG!739 (merged)