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/cluster.go

@@ -15,8 +15,8 @@
 package api
 
 import (
-	"go.etcd.io/etcd/etcdserver/api/membership"
-	"go.etcd.io/etcd/pkg/types"
+	"go.etcd.io/etcd/v3/etcdserver/api/membership"
+	"go.etcd.io/etcd/v3/pkg/types"
 
 	"github.com/coreos/go-semver/semver"
 )