Update scan policy text related to merge request

What does this MR do and why?

Update scan policy text related to merge request creation. From create via merge request to create with a merge request.

Raised as part of this discussion.

Screenshots or screen recordings

Scan result policy

Scan execution policy

How to set up and validate locally

1. Enable scan_result_policy feature flag (only required for scan result policy)
2. Create a security project as described in the docs
3. Create a new policy with the project owner

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.