Trigger `security-report-schemas-ruby` distribution pipeline

What does this MR do?

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#383511 (closed):

• adds the add-to-gem CI job that triggers a downstream pipeline in security-report-schemas-ruby.

Example workflow

See gitlab-org/ruby/gems/gitlab-security_report_schemas!22 (merged)

What are the relevant issue numbers?

• gitlab-org/gitlab#383515 (closed)

Checklist

• Ensure changes can be built upon without requiring a breaking change, see Building for Extensibility.
• Review and add/update tests for this feature/bug.
• Add an entry to the CHANGELOG if required, with the appropriate version. See Classifying Changes.
• Assign the MR to the appropriate person/people for review.

Closes #383511