GetCapacityResponse: promote maximum_volume_size

Kubernetes and several CSI drivers have been using that field for a while now
and no changes to the semantic are expected, therefore the alpha status can and
should be removed.

The minimum_volume_size field is not used by Kubernetes. The semantic is
probably stable, but further practical experience with it might be desirable
before promoting it.

Author: pohly

csi.proto

@@ -988,8 +988,7 @@ message GetCapacityResponse {
   // a maximum size for individual volumes and leave it unset
   // otherwise. COs MAY use it to make decision about
   // where to create volumes.
-  google.protobuf.Int64Value maximum_volume_size = 2
-    [(alpha_field) = true];
+  google.protobuf.Int64Value maximum_volume_size = 2;
 
   // The smallest size that may be used in a
   // CreateVolumeRequest.capacity_range.limit_bytes field