go.mod: use go.etcd.io/etcd/v3 versioning

This change makes the etcd package compatible with the existing Go
ecosystem for module versioning.

Used this tool to update package imports:
  https://github.com/KSubedi/gomove

client/integration/main_test.go

@@ -8,7 +8,7 @@ import (
 	"os"
 	"testing"
 
-	"go.etcd.io/etcd/pkg/testutil"
+	"go.etcd.io/etcd/v3/pkg/testutil"
 )
 
 func TestMain(m *testing.M) {