Update service-elser.asciidoc (elastic#116272)

(cherry picked from commit d1788af)

docs/reference/inference/service-elser.asciidoc

@@ -7,6 +7,12 @@ You can also deploy ELSER by using the <<infer-service-elasticsearch>>.
 NOTE: The API request will automatically download and deploy the ELSER model if
 it isn't already downloaded.
 
+[WARNING]
+.Deprecated in 8.16
+====
+The elser service is deprecated and will be removed in a future release. 
+Use the <<infer-service-elasticsearch>> instead, with model_id included in the service_settings.
+====
 
 [discrete]
 [[infer-service-elser-api-request]]
@@ -173,4 +179,4 @@ PUT _inference/sparse_embedding/my-elser-model
   }
 }
 ------------------------------------------------------------
-// TEST[skip:TBD]
+// TEST[skip:TBD]