*Bug description*
The useradd command will fail when creating a user and explicitly adding the user to a group of the same name.
E.g. `useradd --groups 'wheel,myuser' --password '123456' myuser` will fail with exit status 6, because the 'myuser' group does not exist.

Hence we need to validate this in the frontend (and ideally on the image-builder-crc API layer, too) and not allow adding a group with the same name as the user.

*Reproduction steps*
Steps to reproduce the behavior:
1. Go to the service and create a blueprint
2. In the user step, add a user and add a group with the same name
3. Go to review
4. Build
5. Observe that the build will fail

*Expected behavior*
We should not allow customers to add a group that doesn't exist, in this case the group with the name of the newly-created user.

GitHub Issue: https://github.com/osbuild/image-builder-frontend/issues/3932