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

pkg/netutil/netutil.go

@@ -23,7 +23,7 @@ import (
 	"sort"
 	"time"
 
-	"go.etcd.io/etcd/pkg/types"
+	"go.etcd.io/etcd/v3/pkg/types"
 
 	"go.uber.org/zap"
 )