Distribute Ruby gem

Why is this change being made?

We are adding a RubyGem-based distribution workflow to security-report-schemas.

See [DESIGN] Rubygem-based release for security-report-schemas.

This MR addresses gitlab-org/gitlab#383515 (closed):

• builds and pushes the gem to rubygems.org
• opens an MR to gitlab-org/gitlab to incorporate the newly released gem version

Example workflow

• security-report-schemas deploy-gem job triggers downstream pipeline in security-report-schemas-ruby
• add-schema-version job retrieves schemas, commits them and pushes a tag to its own repository
• release-gem job builds and release the RubyGem (here --dry-run was used)
• open-merge-request job updates the Gemfile and creates a merge request. The MR is missing Gemfile.lock, because without the RubyGem version having been released prior, bundle install will complain about an unresolvable version.