fix: allow explain for all add ons

What does this MR do and why?

This change reverts /explain behavior to be allowed back for duo core, pro, enterprise customers to help unblock #587539 (closed). This work was originally part of the work to gate external triggers for new DAP billing strategy (https://gitlab.com/gitlab-org/gitlab/-/work_items/581209) but there is an incompatibility with how /explain works. This change re-allows /explain to work as originally functioned, and any gating of the UI button for /explain will happen on the frontend in a follow-up

References

#587539 (closed)

How to set up and validate locally

• Check out this branch
• Using a Duo Core account, ensure that /explain works in duo chat

Screenshots

Before	After

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