Update gitlab-backup-cli to support both 7.0 and 7.1

Igor Drozdovrequested to merge
id-update-gitlab-backup-cli-gems into master

What does this MR do and why?

According to Rails Upgrade process Gitlab codebase supports both current and next Rails versions. It's the reason why we had a looser version for the dependency before Update gems for gitlab-backup-cli for version p... (!165322 - merged)

If the compatibility is the concern here, let's run tests for both Rails versions for this dependency similar to Update activerecord-gitlab to support both 7.0 ... (!161441 - merged)

This MR:

  • Introduces Gemfile and Gemfile.next in the gitlab-backup-cli gem
  • Modifies pipeline to execute tests for both versions

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