pkg/userns: deprecate and migrate to github.com/moby/sys/user/userns

The userns package in libcontainer was integrated into the moby/sys/user
module at commit [3778ae603c706494fd1e2c2faf83b406e38d687d][1].

This patch deprecates the containerd fork of that package, and adds it as
an alias for the moby/sys/user/userns package.

[1]: 3778ae603c

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

pkg/sys/oom_linux_test.go

@@ -24,7 +24,7 @@ import (
 	"testing"
 	"time"
 
-	"github.com/containerd/containerd/v2/pkg/userns"
+	"github.com/moby/sys/user/userns"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )