[TypeSpecValidation] Block diagonal conversions

This PR was an unintentional diagonal conversion, that should have been blocked:

https://github.com/Azure/azure-rest-api-specs/pull/34972

The handwritten spec version existed in RPSaaSMaster, but this PR was mistakenly opened against main (which did not have it), which made it a diagonal conversion, which masked a breaking change adding a new required property.

For implementation, I think this logic can be added to check `TypeSpecRequirement`:

* if:
* spec before PR contains only handwritten-swagger, AND
* spec after PR contains typespec-generated swagger, AND
* PR introduces a new API version
* then
* block PR (no suppression available)