Add impact assessment requirement to static analysis check template

What does this MR do and why?

• Add checkbox requiring impact analysis on gitlab-org/gitlab
• Add new "Impact on gitlab-org/gitlab" section with detailed instructions
• Include steps for testing new cops against the monolith repository
• Provide template for documenting rubocop output results

This ensures new static analysis checks are tested against the main GitLab codebase before approval, helping maintainers understand the scope and potential disruption of proposed changes.

Refs

Based on !259 (comment 2671199990)