Switch to Ruby 2.7

What does this MR do?

Ruby 2.5 is fairly old, this bumps it to the most recent version. This makes the extra step to update bunlde obsolete, because it's now part of Ruby.

Furthermore this changes the base image from Debian Stretch to Alpine to Buster Slim which has a much smaller footprint. The bundle audit version stays the same and there should be no functional change at all.

This has been replaced with !41 (merged)

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Auto-DevOps template
  • Job definition example
  • CI Templates
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer