Deep copy an object: retarget net10.0, drop AutoMapper, fix the benchmark

[vladimir-pecanac-main]

Updates the sample for "Deep Copy and Clone an Object in C#" (csharp-intermediate-topics/DeepCopyInCSharp). This PR touches only this folder.

Changes:

1. Both projects retargeted from net7.0 to net10.0.
2. Packages, re-queried on NuGet on 2026-09-23 (newest stable of each):
   • Microsoft.NET.Test.Sdk 17.3.2 to 18.10.1 (the batch block said 18.10.0; 18.10.1 is newer today, so the re-query wins)
   • 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
   • BenchmarkDotNet 0.13.5 to 0.15.8
   • Newtonsoft.Json 13.0.3 to 13.0.4
   • DeepCopy 1.0.3 and FastDeepCloner 1.3.6 unchanged: they are the newest published versions.
3. AutoMapper removed, with its mapper setup in DeepCopyMaker, its benchmark and its test. AutoMapper 12.0.1 raises NU1903 on restore (GHSA-rvv3-g6hj-g44x, CVE-2026-32933, high). Every MIT-licensed version is in the vulnerable range and the patched versions (15.1.1, 16.1.1) are commercially licensed. The article now points readers to the dedicated AutoMapper article instead.
4. Expression trees: DeepCopyExpressionTrees compiled a new delegate on every call, and the generated code compiled again for the nested Address. The delegate is now built once per type in a static generic cache, and the nested call goes through the same cache.
5. ConfigOptions.DisableOptimizationsValidator stays in Program.cs, now with a comment saying why. Removing it was tried: BenchmarkDotNet 0.15.8 then refuses to run at all, because FastDeepCloner 1.3.6 ships an assembly built without optimizations and the validator checks every referenced assembly. The switch covers every assembly, so the comment says to run the project with -c Release, which is how the benchmark was run.
6. Program.cs demonstrations: the shallow copy result used to be overwritten by ten more assignments before anything was printed, so the program could never show the shallow copy output the article prints. Each technique now copies, modifies and prints its own fresh original.
7. Copy constructors added to Person and Address ([SetsRequiredMembers]), with a test asserting the copied Address is a different instance, and a CopyConstructorBenchmark so the article's copy constructor figure comes from the same run as the others.
8. Nullable warnings fixed: ArgumentNullException.ThrowIfNull on the inputs, non-null returns where the serializer contract allows it.

Verified locally on SDK 10.0.302, runtime .NET 10.0.10, Windows 10 (AMD Ryzen 5 3600): dotnet build -c Release, 0 warnings, 0 errors. dotnet test, 10 passed, 0 failed. dotnet list package --vulnerable --include-transitive reports no vulnerable packages in either project. The benchmark was re-run in Release on the same machine.