user v0.2.0

⚠️ this release was retracted

After further discussion with maintainers, it was decided to move the
userns package to a separate module as it has no direct relation with
"user" operations (other than having "user" in its name).

Refer to the user v0.3.0 and userns v0.1.0 release notes for details.

What's Changed

• user: require go1.21 or higher

• integrate libcontainer/userns into moby/sys/user

  This release integrates the userns package from libcontainer (runc) at commit;
  3778ae603c706494fd1e2c2faf83b406e38d687d.

  Code was originally added through runc@c0ad40c, and located in the
  libcontainer/system package; history of this code from before it was moved
  can be found through 3 and 4.

Full Changelog: user/v0.1.0...user/v0.2.0