Update rest-api-spec

Author: github-actions[bot]

output/schema/validation-errors.json

@@ -28,6 +28,12 @@
       ]
     },
     "params": {
+      "force_synthetic_source": {
+        "type": "boolean",
+        "description": "Should this request force synthetic _source? Use this to test if the mapping supports synthetic _source and to get a sense of the worst case performance. Fetches with this enabled will be slower the enabling synthetic source natively in the index.",
+        "visibility": "feature_flag",
+        "feature_flag": "es.index_mode_feature_flag_registered"
+      },
       "stored_fields": {
         "type": "list",
         "description": "A comma-separated list of stored fields to return in the response"