Hardcode security report schemas version to v15.0.7

What does this MR do?

This merge request hardcodes the Security Report Schemas version to v15.0.7 as newer versions are indigestible for Gitlab 16.x versions, since the corresponding merge request to update this for Rails project was only merged after 16.11 is released.

Please check this comment from a customer who's affected by this.

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 updated/added for this feature/bug
• Job definition updated, if necessary
  • Auto-DevOps template
  • Job definition example
  • CI Templates
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer