Replace Hard-coded HTTP Verbs with Constants (#27219)

* Replace hard-coded HTTP verbs with constants in `net/http`

server/channels/api4/elasticsearch.go

@@ -13,8 +13,8 @@ import (
 )
 
 func (api *API) InitElasticsearch() {
-	api.BaseRoutes.Elasticsearch.Handle("/test", api.APISessionRequired(testElasticsearch)).Methods("POST")
-	api.BaseRoutes.Elasticsearch.Handle("/purge_indexes", api.APISessionRequired(purgeElasticsearchIndexes)).Methods("POST")
+	api.BaseRoutes.Elasticsearch.Handle("/test", api.APISessionRequired(testElasticsearch)).Methods(http.MethodPost)
+	api.BaseRoutes.Elasticsearch.Handle("/purge_indexes", api.APISessionRequired(purgeElasticsearchIndexes)).Methods(http.MethodPost)
 }
 
 func testElasticsearch(c *Context, w http.ResponseWriter, r *http.Request) {