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

pkg/adt/example_test.go

@@ -17,7 +17,7 @@ package adt_test
 import (
 	"fmt"
 
-	"go.etcd.io/etcd/pkg/adt"
+	"go.etcd.io/etcd/v3/pkg/adt"
 )
 
 func Example() {