Support .NET 9.0 CountBy

[Release notes](https://learn.microsoft.com/en-us/dotnet/core/whats-new/dotnet-9/overview#linq)
[Docs](https://learn.microsoft.com/en-us/dotnet/api/system.linq.enumerable.countby?view=net-9.0)

```c#
KeyValuePair mostFrequentWord = sourceText
.Split(new char[] { ' ', '.', ',' }, StringSplitOptions.RemoveEmptyEntries)
.Select(word => word.ToLowerInvariant())
.CountBy(word => word)
.MaxBy(pair => pair.Value);
```