Set request body to required to reflect the code base (#27188)

Update API, Cluster Update Settings API and Put Index Template API didn't have the request body set to required in their spec, hence this commit updates the spec to align them with reality.

rest-api-spec/src/main/resources/rest-api-spec/api/cluster.put_settings.json

@@ -22,7 +22,8 @@
       }
     },
     "body": {
-      "description": "The settings to be updated. Can be either `transient` or `persistent` (survives cluster restart)."
+      "description": "The settings to be updated. Can be either `transient` or `persistent` (survives cluster restart).",
+      "required" : true
     }
   }
 }

rest-api-spec/src/main/resources/rest-api-spec/api/search_template.json

@@ -62,7 +62,8 @@
       }
     },
     "body": {
-      "description": "The search definition template and its params"
+      "description": "The search definition template and its params",
+      "required" : true
     }
   }
 }

rest-api-spec/src/main/resources/rest-api-spec/api/update.json

@@ -88,7 +88,8 @@
       }
     },
     "body": {
-      "description": "The request definition using either `script` or partial `doc`"
+      "description": "The request definition requires either `script` or partial `doc`",
+      "required": true
     }
   }
 }