fix: retract tagged versions (#590)

Retract unpublished versions from go tooling.
Bump go version to 1.16 required for retract statement.

Fixes #585

go.mod

@@ -1,5 +1,10 @@
 module github.com/gomodule/redigo
 
-go 1.14
+go 1.16
 
 require github.com/stretchr/testify v1.7.0
+
+retract (
+	v2.0.0+incompatible // Old development version not maintained or published.
+	v0.0.0-do-not-use // Never used only present due to lack of retract.
+)