Forbid removing the last admin user (#28337)

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

templates/swagger/v1_json.tmpl

@@ -677,6 +677,9 @@
           "200": {
             "$ref": "#/responses/User"
           },
+          "400": {
+            "$ref": "#/responses/error"
+          },
           "403": {
             "$ref": "#/responses/forbidden"
           },