cloudapi: reduce minsize type to just string

The type of the minsize parameter in the new disk customization was
meant to support both integers (size in bytes) and strings (size with
unit).  However, the schema wasn't done properly, which made any input
result in an error:

    GenericError: Failed to create the compose request:
    {"code":"IMAGE-BUILDER-COMPOSER-30","details":"request body has an
    error: doesn't match schema #/components/schemas/ComposeRequest:
    Error at \"/customizations/disk/partitions/0\": doesn't match schema
    due to: Error at \"/minsize\": input matches more than one oneOf
    schemas

Reducing it to just support strings simplifies the schema.  It's also
not an important feature reduction since sizes as integers (for
filesystems that are typically in GiB) aren't very convenient.

internal/cloudapi/v2/compose.go

@@ -1429,13 +1429,5 @@ func decodeMinsize(size *Minsize) (uint64, error) {
 		return 0, nil
 	}
 
-	if sizeStr, err := size.AsMinsize1(); err == nil {
-		return datasizes.Parse(sizeStr)
-	}
-
-	if sizeUint, err := size.AsMinsize0(); err == nil {
-		return sizeUint, err
-	}
-
-	return 0, fmt.Errorf("failed to convert value \"%v\" to number", size)
+	return datasizes.Parse(*size)
 }