docs: add note about quiet flag (#1518)

migration-guide.md

@@ -86,7 +86,7 @@ The following table lists in alphabetical order v1 flags and their v2 version.
 | log_debug_stdout | ❌ | v2 logs to stdout, errors to stderr by default |
 | max_connections | max-connections | |
 | projects | ❌ | v2 prefers explicit connection configuration to avoid user error |
-| quiet | ❌  |  |
+| quiet | quiet | quiet disables all logging except errors |
 | quota_project | quota-project | |
 | refresh_config_throttle | ❌ | |
 | skip_failed_instance_config | ❌ | |