Restrict Self-Managed Code Suggestions to paid tiers

What does this MR do and why?

Context: https://gitlab.com/gitlab-org/gitlab/-/issues/434686. Epic: https://gitlab.com/groups/gitlab-org/-/epics/11996.

Add a restriction to code_suggestions_helper.rb that ensures self-managed instances have a paid license before allowing them to access Code Suggestions in the Admin area.

Screenshots or screen recordings

No visual changes.

How to set up and validate locally

CI pipeline should continue to pass ✅

MR acceptance checklist

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