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

tools/etcd-dump-metrics/main.go

@@ -25,7 +25,7 @@ import (
 	"path/filepath"
 	"time"
 
-	"go.etcd.io/etcd/embed"
+	"go.etcd.io/etcd/v3/embed"
 
 	"go.uber.org/zap"
 )