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

proxy/grpcproxy/adapter/lock_client_adapter.go

@@ -17,7 +17,7 @@ package adapter
 import (
 	"context"
 
-	"go.etcd.io/etcd/etcdserver/api/v3lock/v3lockpb"
+	"go.etcd.io/etcd/v3/etcdserver/api/v3lock/v3lockpb"
 
 	"google.golang.org/grpc"
 )