DRA: remove "source" indirection from v1 Pod API

This makes the API nicer:

    resourceClaims:
    - name: with-template
      resourceClaimTemplateName: test-inline-claim-template
    - name: with-claim
      resourceClaimName: test-shared-claim

Previously, this was:

    resourceClaims:
    - name: with-template
      source:
        resourceClaimTemplateName: test-inline-claim-template
    - name: with-claim
      source:
        resourceClaimName: test-shared-claim

A more long-term benefit is that other, future alternatives
might not make sense under the "source" umbrella.

This is a breaking change. It's justified because DRA is still
alpha and will have several other API breaks in 1.31.

test/e2e_node/dra_test.go

@@ -502,10 +502,8 @@ func createTestObjects(ctx context.Context, clientSet kubernetes.Interface, node
 			NodeName: nodename, // Assign the node as the scheduler is not running
 			ResourceClaims: []v1.PodResourceClaim{
 				{
-					Name: podClaimName,
-					Source: v1.ClaimSource{
-						ResourceClaimName: &claimName,
-					},
+					Name:              podClaimName,
+					ResourceClaimName: &claimName,
 				},
 			},
 			Containers: []v1.Container{