Move explain code prompt to the backend
<!--IssueSummary start--> <details> <summary> Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards. </summary> - [Work on this issue](https://contributors.gitlab.com/manage-issue?action=work&projectId=278964&issueIid=407669) - [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=407669) </details> <!--IssueSummary end--> We recently delivered the first iteration of the Explain code with AI feature. As part of the first iteration, we decided to generate the prompt on the [frontend](https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/app/assets/javascripts/ai/utils.js). This feature suggests moving the prompt to the backend.
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