Grafana: Replace magic number with a constant variable in response status (#80132)

* Chore: Replace response status with const var

* Apply suggestions from code review

Co-authored-by: Sofia Papagiannaki <1632407+papagian@users.noreply.github.com>

* Add net/http import

---------

Co-authored-by: Sofia Papagiannaki <1632407+papagian@users.noreply.github.com>

pkg/api/routing/routing.go

@@ -1,6 +1,8 @@
 package routing
 
 import (
+	"net/http"
+
 	"github.com/grafana/grafana/pkg/api/response"
 	contextmodel "github.com/grafana/grafana/pkg/services/contexthandler/model"
 	"github.com/grafana/grafana/pkg/web"
@@ -8,7 +10,7 @@ import (
 
 var (
 	ServerError = func(err error) response.Response {
-		return response.Error(500, "Server error", err)
+		return response.Error(http.StatusInternalServerError, "Server error", err)
 	}
 )