Change to go 1.18
Change to go 1.18

Signed-off-by: Laurent Grangeau <919068a1571bc1b5deaaf9ac4d631597f8dd629f@gmail.com>

GoogleCloudPlatform/gke-fqdnnetworkpolicies-golang,GoogleCloudPlatform/gke-fqdnnetworkpolicies-golang