ConvertApiUser.SecondsLeft property is depreciated

ConvertApiUser.SecondsLeft is depreciated. Remove property and replace with `ConversionsTotal `and `ConversionsConsumed`
```
ConvertApiUser user = await convert.GetUserAsync();
int secondsLeft = user.SecondsLeft;
```

After updating fix description and code https://github.com/ConvertAPI/convertapi-dotnet/blob/6281590e92ff09e81689feabde4594533998662a/README.md?plain=1#L95