schema: wrong validation of cc_users_groups

[Alberto Contreras]

The schema validation does detect the following wrong schema as valid:

```yaml
#cloud-config
groups:
  - 3.14
```

Steps to reproduce:

```sh
cat <<EOF > /tmp/user-data
#cloud-config
groups:
  - 3.14
EOF

lxc launch ubuntu-daily:jammy gg --config=user.user-data="$(cat /tmp/user-data)"
$ lxc exec gg -- cloud-init status --wait
..............
status: error

$ lxc exec gg -- grep -ni "error" /var/log/cloud-init.log
412: raise TypeError(
413:TypeError: Unknown group name type float
440: raise TypeError(
441:TypeError: Unknown group name type float

$ lxc exec gg -- cloud-init schema --system
Valid cloud-config: system userdata

$ lxc exec gg -- echo $?
0
```

The schema validation should detect this schema as invalid.

[Alberto Contreras (aciba)]

Attach logs

[James Falcon (falcojr)]

Tracked in Github Issues as https://github.com/canonical/cloud-init/issues/3991