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

raft/interaction_test.go

@@ -18,7 +18,7 @@ import (
 	"testing"
 
 	"github.com/cockroachdb/datadriven"
-	"go.etcd.io/etcd/raft/rafttest"
+	"go.etcd.io/etcd/v3/raft/rafttest"
 )
 
 func TestInteraction(t *testing.T) {