api: drop Resources.Claims from PVC and PVC template

PVC and containers share the same ResourceRequirements struct. The Claims field
in it only makes sense when used in containers. When used in a PVC, the field
should have been rejected by validation. This was overlooked when introducing
it, so now persisted objects might have it set and/or people may have started
to rely on it being accepted even when it has no effect.

Therefore we cannot reject it in validation anymore, but we can still strip
it out on create or update.

Kubernetes-commit: bb8e051b4b95e1eb0e2dc4e3e25b4373a8a6307d

Author: pohly

core/v1/types.go

@@ -2320,7 +2320,7 @@ type ResourceRequirements struct {
 	// This is an alpha field and requires enabling the
 	// DynamicResourceAllocation feature gate.
 	//
-	// This field is immutable.
+	// This field is immutable. It can only be set for containers.
 	//
 	// +listType=map
 	// +listMapKey=name