Update Security Reporting Policy

[Firefishy]

• [x] Move https://github.com/openstreetmap/operations/blob/master/SECURITY.md to https://operations.osmfoundation.org/policies/
• [ ] Add SECURITY.md to project repos, referencing Security Policy at https://operations.osmfoundation.org/policies/
• [ ] Add a security.txt to primary sites referencing Security Policy at https://operations.osmfoundation.org/policies/