rest-api-spec: mark search_mvt as stable (#133915)

Author: pquentin

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

@@ -4,7 +4,7 @@
       "url": "https://www.elastic.co/docs/api/doc/elasticsearch/operation/operation-search-mvt",
       "description": "Search a vector tile"
     },
-    "stability": "experimental",
+    "stability": "stable",
     "visibility": "public",
     "headers": {
       "accept": [