Drop support for Ruby 2.5
Problem
According to https://www.ruby-lang.org/en/downloads/branches/ Ruby 2.5 only receives security patches:
Ruby 2.5
status: security maintenance
release date: 2017-12-25
EOL date: 2021-03-31
Solution
Drop support for Ruby 2.5. The closest GitLab milestone is %13.11 (see https://gitlab.com/groups/gitlab-org/-/milestones).