Update rest-api-spec (#4041)

Co-authored-by: pquentin <[email protected]>

Author: github-actions[bot]

output/openapi/elasticsearch-openapi.json

@@ -34,6 +34,11 @@
         "description": "Whether the index template should only be added if new or can also replace an existing one",
         "default": false
       },
+      "cause": {
+        "type": "string",
+        "description": "User defined reason for creating/updating the index template",
+        "default": ""
+      },
       "master_timeout": {
         "type": "time",
         "description": "Specify timeout for connection to master"