go.mod: bump Go features to 1.16

For example, this enables the use of the vendor
 directory by
default (https://golang.org/ref/mod#vendoring).

Author: pohly

go.mod

@@ -1,6 +1,6 @@
 module github.com/kubernetes-csi/csi-driver-host-path
 
-go 1.12
+go 1.16
 
 require (
 	github.com/container-storage-interface/spec v1.3.0