Update summarize merge request to be self managed

What does this MR do and why?

• Follow-up of this MR: Add unit primitive and permission checks for su... (!185430 - merged)
• This MR updates summarize merge request feature to be self-managed

Verifications:

• Unit primitive for the summarize_new_merge_request feature already exists in the gitlab-cloud-connector repo: summarize_new_merge_request.yml ✅
• There is no need to add an entry in access_data.yml given it's deprecated ✅

How to set up and validate locally

Prerequisites

• Make sure you've setup local AI features by following this guide

Steps

• Run GDK with AI features locally
• Create (or open an existing) MR
• Edit description and make sure Summarize code changes button is accessible and working as expected

Screenshot

• Make sure the summarize merge request feature is still working as expected

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #497503 (closed)