Remove whitespace from a string: retarget net10.0, bump packages

[vladimir-pecanac-main]

Retargets all three projects in strings-csharp/RemoveWhitespaceCharactersFromString from net7.0 to net10.0 and bumps every package. No source changes: the sample was already correct, and the fixes this article needs are on the article's side.

Packages, re-queried on NuGet on 2026-09-23, the day this PR opened:

• BenchmarkDotNet 0.13.5 to 0.15.8
• Bogus 34.0.2 to 35.6.5
• Microsoft.NET.Test.Sdk 17.5.0 to 18.10.1 (the batch plan named 18.10.0; 18.10.1 is today's newest stable, 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 3.2.0 to 10.0.1

Checked locally on SDK 10.0.302, runtime 10.0.10:

• dotnet build -c Release: 0 warnings, 0 errors
• dotnet test: 97 passed, 0 failed (11 test methods, theory data expands them to 97 cases)
• dotnet list package --vulnerable --include-transitive: no vulnerable packages in any of the three projects