backstage/.changeset/3113.md

@backstage/catalog-model, @backstage/plugin-catalog-backend

@backstage/catalog-model	@backstage/plugin-catalog-backend
minor	minor

Changes the various kind policies into a new type KindValidator.

Adds CatalogProcessor#validateEntityKind that makes use of the above validators. This moves entity schema validity checking away from entity policies and into processors, centralizing the extension points into the processor chain.