Retract pre go module OPA versions (#5741)

We have two versions of our go package docs showing in https://pkg.go.dev/.

A) open-policy-agent/OPA &
B) https://pkg.go.dev/github.com/open-policy-agent/opa

A appears to contain only versions up to 0.15.0 where we started using
go modules. B is the correct one we want to keep using.

According to advice here: golang/go#58925, we
need to retract the old versions to have this page disappear. Given the
time that has passed since the 0.15.0 release I hope we can make this
change now.

Signed-off-by: Charlie Egan <[email protected]>

Author: Charlie Egan

go.mod

@@ -96,3 +96,7 @@ require (
 // at a cost, its init function will lookup the current user, and on Windows, that's much
 // work.
 replace github.com/golang/glog => ./build/replacements/github.com/golang/glog
+
+// Before 0.15.0 OPA was not using go modules. We must retract these releases in order to have pkg.go.dev
+// show the latest versions. See: https://github.com/golang/go/issues/58925
+retract [v0.1.0-rc1, v0.14.2]