Remove resolve_vulnerability_in_mr flag

What does this MR do and why?

This merge request makes the "Vulnerability Resolution with AI" feature generally available in GitLab 17.11. The feature, which automatically creates merge request suggestions to resolve security vulnerabilities using Anthropic's Claude 3.5 Sonnet model, was previously introduced in GitLab 17.6, enabled by default in 17.7, and is now fully released.

The changes include:

• Removing the feature flag resolve_vulnerability_in_mr since the feature is now generally available
• Updating documentation to reflect the feature's general availability status
• Removing feature flag checks from the code, making the AI resolution capability permanently available
• Updating tests to remove references to the now-removed feature flag

References

Screenshots or screen recordings

description	screenshot
AI-badge for resolvable findings
Info-popup for public projects
Info-popup for private projects
AI-MR comment

How to set up and validate locally

MR acceptance checklist

1. Follow the instructions from tthis README to set up an MR with AI-resolvable findings
2. Verify that a duo-badge shows for findings that have an AI-resolution available
3. Click on a finding
4. If the project is private - it should not show up a warning message in the button's pop-up
5. If the project is public - it should show up a warning message in the button's pop-up
6. Resolve the finding and make sure that a comment with a link to the created MR is added
7. Check the MR that was created and make sure the description, etc. is correct

Related to #525066 (closed)