Bumping ruby version to 3.0.0 (except docker/gitlab/Dockerfile)

Why?:

Since ruby 2.7 EOL is nearing, it's good to bump it up to 3.0.0.

What it does?:

This MR update the ruby version only in .tool-versions and .gitlab-ci.yml, which will close the issue #77 (closed).

Updating the Docker image in "gitlab/Dockerfile" will be handled separately as part of #78.