chore(php): set Compute migration_mode to NEW_SURFACE_ONLY (#1101)

Author: bshaffer

google/cloud/compute/v1/BUILD.bazel

@@ -287,7 +287,7 @@ php_gapic_library(
     grpc_service_config = "compute_grpc_service_config.json",
     service_yaml = "compute_v1.yaml",
     transport = "rest",
-    migration_mode = "MIGRATING",
+    migration_mode = "NEW_SURFACE_ONLY",
     deps = [
         ":compute_php_proto",
     ],