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

clientv3/integration/watch_fragment_test.go

@@ -23,9 +23,9 @@ import (
 	"testing"
 	"time"
 
-	"go.etcd.io/etcd/clientv3"
-	"go.etcd.io/etcd/integration"
-	"go.etcd.io/etcd/pkg/testutil"
+	"go.etcd.io/etcd/v3/clientv3"
+	"go.etcd.io/etcd/v3/integration"
+	"go.etcd.io/etcd/v3/pkg/testutil"
 )
 
 // TestWatchFragmentDisable ensures that large watch