Add version information for Ruby version managers to project
For folks using Ruby version managers, sets preferred version for project.
Set to match what we use in gitlab
: https://gitlab.com/gitlab-org/gitlab/-/blob/97ee889c8d7ac00051ad6c860ef538280490fc5e/.ruby-version and CI/CD: https://gitlab.com/gitlab-org/quality/triage-ops/-/blob/d94f422ab4ab500f5bca6fd72fa834ee24f2d5b1/.gitlab-ci.yml#L25.