go.mod: bump Go features to 1.16

pohly · pohly · commit 9ede1ed02e1d · 2021-03-18T09:27:08.000+01:00
For example, this enables the use of the vendor directory by default (https://golang.org/ref/mod#vendoring).
diff --git a/go.mod b/go.mod
@@ -1,6 +1,6 @@
 module github.com/kubernetes-csi/external-snapshotter/v4
 
-go 1.15
+go 1.16
 
 require (
 	github.com/container-storage-interface/spec v1.3.0
