Figure out a removal flow for security report schemas

Problem

After we deprecate a schema, at some point we must remove it. It appears we can just delete the schema file from the repository and be done with it but perhaps a cop as suggested in #350993 (closed) would prevent accidental removals of deprecated-but-still-supported or supported schemas.

Implementation plan

1. backend Create a spec that verifies that all supported and deprecated schema versions are present on the disk