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>

core/mount/mount_linux.go

@@ -28,8 +28,8 @@ import (
 	"strings"
 	"time"
 
-	"github.com/containerd/containerd/v2/pkg/userns"
 	"github.com/containerd/log"
+	"github.com/moby/sys/user/userns"
 	"golang.org/x/sys/unix"
 )