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

functional/tester/case_failpoints.go

@@ -21,7 +21,7 @@ import (
 	"strings"
 	"sync"
 
-	"go.etcd.io/etcd/functional/rpcpb"
+	"go.etcd.io/etcd/v3/functional/rpcpb"
 )
 
 type failpointStats struct {