docs: breaking change announcement

Author: necusjz

src/azure-cli/azure/cli/command_modules/network/_breaking_change.py

@@ -2,3 +2,8 @@
 # Copyright (c) Microsoft Corporation. All rights reserved.
 # Licensed under the MIT License. See License.txt in the project root for license information.
 # --------------------------------------------------------------------------------------------
+from azure.cli.core.breaking_change import register_argument_deprecate
+
+
+register_argument_deprecate('az network vnet subnet create', '--default-outbound', target_version='2.73.0')
+register_argument_deprecate('az network vnet subnet update', '--default-outbound', target_version='2.73.0')