Significantly improve .NET's code

[OoLunar]

Significantly improves .NET's code, closes https://github.com/pkolaczk/async-runtimes-benchmarks/pull/4

• Updated to .NET 7
• Dropped ImplicitUsings in dotnet.csproj
• Uses an array (Task[]) over a list (List<Task>) to prevent constant resizing of the internally backed array.
• Dropped Task.Run and passes the Task itself in the for loop. This prevents the unnecessary creation of an additional async contexts.

Additionally ValueTasks are typically used in hotpaths over Task's due to their newer implementation and don't suffer the scars of .NET framework.