CI: Run Ruby syntax checks only on Ruby related changes

What does this MR do and why?

This MR ensure that Ruby syntax check CI jobs are run on Ruby code changes.

For example, https://gitlab.com/gitlab-community/gitlab/-/pipelines/1160951727 did run these jobs although this MR only changed docs.

Follow-up of !140332 (merged).

Thanks @Taucher2003 for spotting this in Slack 🙇

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

No Ruby code changed https://gitlab.com/gitlab-org/gitlab/-/pipelines/1160979094	Ruby code changed https://gitlab.com/gitlab-org/gitlab/-/pipelines/1161013848
No CI jobs run ✅	CI jobs run ✅