Task.CompletedTask vs Task.FromResult vs return: retarget to net10.0 and drop the redundant async

[vladimir-pecanac-main]

Updates async-csharp/TaskCompletedTaskFromResultVsReturnInCSharp for the article rewrite.

• Both projects retargeted from net7.0 to net10.0.
• Test packages: Microsoft.NET.Test.Sdk 17.3.2 to 18.10.1, xunit 2.4.2 to 2.9.3, xunit.runner.visualstudio 2.4.5 to 4.0.0, coverlet.collector 3.1.2 to 10.0.1 (all re-queried on the NuGet flat-container index today; Test.Sdk 18.10.1 is newer than the 18.10.0 the plan named).
• UseTaskFromResultAsync() no longer writes 'return await Task.FromResult(message)' inside an async method. It drops async and await and returns the finished task directly, which is half the allocation for the same result.
• UseTaskCompletedSync() and UseTaskFromResultSync() are removed, together with their calls in Program.cs. Neither demonstrated blocking: both called Wait() or .Result on a task that was already complete.
• Adds INotificationHandler and ConsoleNotificationHandler, the real case for Task.CompletedTask, where the signature is imposed by an interface and there is nothing to await.
• Adds two tests asserting that the task handed back by a non-async method is already complete without being awaited. Three tests become five.

Build: 0 errors, 0 warnings. Tests: 5 of 5 passing on SDK 10.0.302 / runtime 10.0.10.