Cut and release security report schema version 15-0-0

Why are we doing this work

As discussed and agreed in #361288 (closed), the next major version of the security-report-schemas is to be released as part of GitLab %15.4.

Relevant links

Non-functional requirements

• Documentation:
• Feature flag:
• Performance:
• Testing:

Implementation plan

• Merge gitlab-org/security-products/security-report-schemas!100 (merged)
• Vendor new version in gitlab-org/gitlab

Verification steps

1. Ingest a security report that uses schema version 15.0.0 in a new CI pipeline
2. Ensure there are no errors or warnings in the CI pipeline security tab.
3. Run Package and QA before MR is merged and check whether existing secure tests pass. The schema version in the pre-made reports used in E2e tests is 14.1.1