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

tests/e2e/cluster_proxy_test.go

@@ -24,7 +24,7 @@ import (
 	"strconv"
 	"strings"
 
-	"go.etcd.io/etcd/pkg/expect"
+	"go.etcd.io/etcd/v3/pkg/expect"
 )
 
 type proxyEtcdProcess struct {