Adds Gitlab::Saas.enabled? to the avoid instance checks rubocop

What does this MR do and why?

Adds Gitlab::Saas.enabled? to the avoid instance checks rubocop

• this method was never intended to be called outside of the module, but has been. So we'll add it here to encourage proper use as documented in https://docs.gitlab.com/ee/development/ee_features.html#saas-only-feature

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #431302 (closed)