There is a bracking change in v5. Consider this guide: https://github.com/microsoftgraph/msgraph-sdk-dotnet/blob/main/docs/upgrade-to-v5.md

Now this call should be like:

//var message = ....
//var saveToSentItems = ...

var body = new Microsoft.Graph.Me.SendMail.SendMailPostRequestBody
{
    Message = message,
    SaveToSentItems = saveToSentItems
};

await graphServiceClient.Me
    .SendMail
    .PostAsync(body);