feat: Remove cases state handling from /cases endpoints (planning#511, planning#512)

spec/callbacks/case-closure-announcement.yaml

spec/endpoints/cases/index.yaml

spec/endpoints/cases/uuid.yaml

spec/endpoints/destinations/index.yaml

spec/endpoints/destinations/uuid.yaml

spec/endpoints/submissions/index.yaml

spec/examples/case/case-closed.yaml

spec/examples/case/case.yaml

spec/examples/case/list-cases.yaml

spec/examples/case/patch-case-active.yaml

spec/examples/case/patch-case-closed.yaml

...schemas/callbacks/callback-case-closure-announcement.yaml

spec/schemas/case/case-for-closure.yaml

spec/schemas/case/case-state.yaml

spec/schemas/case/case.yaml

spec/schemas/case/list-cases.yaml

spec/schemas/case/patch-case.yaml

spec/submission-api.yaml