Align max length value of Vulnerability.Description with Report schema

What does this MR do?

Currently, the Vulnerability.Description property's value is getting truncated to 400 characters. However, some rules contain verbose descriptions like this or this one that explains the vulnerability in detail for customers to better understand it. These rules are getting impacted by the 400 character truncation limit.

This MR updates the truncation limit to 1048576 which is defined as max-length for the same property(message) in v15.x Security Report Schema.

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Auto-DevOps template
  • Job definition example
  • CI Templates
• Ensure the report version matches the equivalent schema version
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer