Update motd/00-header to use os-release

[Corentin Noël]

The now recommended way to specify an OS version is in /usr/lib/os-release that is shipped with Ubuntu since a while now.
The lsb-release utility has now long switched to using os-release files https://salsa.debian.org/debian/lsb/-/commit/b05d8b6b70e5af820ddb4e651868488ed9257c17

It would be better for the motd header to look like this:

```
[ -r /usr/lib/os-release ] && . /usr/lib/os-release

if [ -z "$PRETTY_NAME" ] && [ -x /usr/bin/lsb_release ]; then
 # Fall back to using the very slow lsb_release utility
 PRETTY_NAME=$(lsb_release -s -d)
fi

printf "Welcome to %s (%s %s %s)\n" "$PRETTY_NAME" "$(uname -o)" "$(uname -r)" "$(uname -m)"
```