Update simple Ubuntu VM sample to remove references to EOL Ubuntu 18.04

[Simple Linux Virtual Machine](../tree/master/quickstarts/microsoft.compute/vm-simple-linux)

### Issue Details

While the default OS version for this sample references Ubuntu 20.04, Ubuntu 18.04 is still listed as an option.

### Other tasks that can be addressed while fixing this

- [ ] Update API versions.
- [x] Fix subnet anti-pattern (#106002)

Start in quickstarts/microsoft.compute/vm-simple-linux and inspect the sample files for references to Ubuntu 18.04. Remove the obsolete option while preserving the Ubuntu 20.04 default, then verify that the sample no longer lists Ubuntu 18.04; API version updates are listed as an additional unchecked task.

Written by the indexing model from the issue text.