Document 409 error returned by repos/migrate api (#19376)

* Document 409 error returned by repos/migrate api

* Generate swagger spec

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

templates/swagger/v1_json.tmpl

@@ -2222,6 +2222,9 @@
           "403": {
             "$ref": "#/responses/forbidden"
           },
+          "409": {
+            "description": "The repository with the same name already exists."
+          },
           "422": {
             "$ref": "#/responses/validationError"
           }