Remove support for Ruby 2.7.5

We should upgrade the Ruby version in .ruby-version and maybe also just remove 2.7.5 in the CI build matrix.

This is showing up in the vulnerability report in https://gitlab.com/gitlab-com/gl-security/appsec/tooling/dependency-forks/tanuki_emoji-0.7.0-ruby-d10df452/-/security/vulnerability_report

https://gitlab.com/gitlab-com/gl-security/appsec/tooling/dependency-forks/tanuki_emoji-0.7.0-ruby-d10df452/-/security/vulnerabilities/91791268