production: remove old .NET6 snaps

On portneuf, after we're confident that .NET10 hasn't broken things, we can remove the .NET6 snaps.

```
wrigjaso@portneuf:/snap$ snap list --all | grep dotnet
dotnet-sdk                 6.0.428                         253    6.0/stable       canonical**  classic
```

I suspect `snap remove dotnet-sdk` is the command to start with.