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

clientv3/concurrency/example_election_test.go

@@ -21,8 +21,8 @@ import (
 	"sync"
 	"time"
 
-	"go.etcd.io/etcd/clientv3"
-	"go.etcd.io/etcd/clientv3/concurrency"
+	"go.etcd.io/etcd/v3/clientv3"
+	"go.etcd.io/etcd/v3/clientv3/concurrency"
 )
 
 func ExampleElection_Campaign() {