Updates ruby version 2.5 to 3.0 (lib/gitlab/ci/templates/Ruby.gitlab-ci.yml)

Updates ruby version 2.5 to the latest (3.0 or 3.0.1) since Ruby 2.5 was just EOL'ed on 2021-03-31.

Most parts of code were already updated in #327579 (closed).

Note: Out of scope from Community contribution since these are NOT disclosed even for Core Team