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

etcdserver/api/rafthttp/msg_codec_test.go

@@ -19,7 +19,7 @@ import (
 	"reflect"
 	"testing"
 
-	"go.etcd.io/etcd/raft/raftpb"
+	"go.etcd.io/etcd/v3/raft/raftpb"
 )
 
 func TestMessage(t *testing.T) {