Update README to account for Ruby 3

On 2023-03-07, GitLab.com will be updated to use Ruby 3. this commit
updaets the software stack section on the README file to account for
that.

Related to https://gitlab.com/gitlab-com/gl-infra/production/-/issues/5494

README.md

@@ -82,7 +82,7 @@ Instructions on how to start GitLab and how to run the tests can be found in the
 GitLab is a Ruby on Rails application that runs on the following software:
 - Ubuntu/Debian/CentOS/RHEL/OpenSUSE
-- Ruby (MRI) 2.7.7
+- Ruby (MRI) 3.0.5
 - Git 2.33+
 - Redis 5.0+
 - PostgreSQL 12+