Change to use code suggestions helper in the SM APIs

What does this MR do and why?

In this MR we change to use the code suggestions helper methods

• Introduce a new helper for Gitlab subscriptions
• Guard self-managed APIs with the code_suggestions_available?
• Remove the code_sugegstiois feature and use the gitlab_saas_subscriptions
• Adjust specs

How to set up and validate locally

Green specs are enough ✅

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 #433011