Update go.mod to 1.19 spec

This is not related to the Go version being used, but the min
version a developer can use to contribute.

Signed-off-by: Alex Ellis (OpenFaaS Ltd) <alex@openfaas.com>

Author: alexellis

gateway/go.mod

@@ -1,6 +1,6 @@
 module github.com/openfaas/faas/gateway
 
-go 1.18
+go 1.19
 
 require (
 	github.com/docker/distribution v2.8.1+incompatible