Build and Release MODEL version 15.0.0

What does this MR do?

This MR includes the changes that will be included in the next MODEL release for the Security schema, version 15.0.0.

This is a long-lived MR. PLEASE DO NOT MERGE THIS MR until:

• There is consensus on the changes being made by relevant stakeholders, and
• The changes in this schema is considered ready for release.

This is important as there is no defined process to to iterate on previous MODEL releases when a new MODEL release is out. By merging this MR, we'll be agreeing that unless there are exceptional circumstances there will be no changes made to the 14.x.x schema.

What are the relevant issue numbers?

• gitlab-org/gitlab#339812 (closed)
• gitlab-org/gitlab#361288 (closed)
• gitlab-org/gitlab#361289 (closed)

Checklist

• Ensure changes can be built upon without requiring a breaking change, see Building for Extensibility.
• Review and add/update tests for this feature/bug.
• Add an entry to the CHANGELOG if required, with the appropriate version. See Classifying Changes.
• Assign the MR to the appropriate person/people for review.