Custom naming policy: retarget net10.0, consolidate four projects to two, NUnit 4

[vladimir-pecanac-main]

Sample update for the republished article "JsonNamingPolicy in C#: Built-in and Custom Naming Policies" (/csharp-custom-naming-policy-for-json/).

What changed

• net7 -> net10.0 on both surviving projects. The moniker was the non-canonical net7, and it is out of support either way.
• Four projects become two. CustomNamingPolicy.NET8 (the built-in-policy demo) and CustomNamingPolicy.NET8.Test (its four tests) merge into CustomNamingPolicy and Test, and the solution loses their two entries. The split existed only because the .NET 8 policies would not compile on net7; at net10.0 that reason is gone. CustomNamingPolicy.NET8.Test.csproj carried no ProjectReference at all - its tests exercise JsonNamingPolicy statics from the BCL - so nothing depended on the removed projects.
• Test packages to current stable (NuGet, read at prep time): NUnit 4.6.1, NUnit3TestAdapter 6.3.0, NUnit.Analyzers 4.14.0, Microsoft.NET.Test.Sdk 18.9.0, coverlet.collector 10.0.1. The seven Assert.AreEqual calls move to Assert.That(actual, Is.EqualTo(expected)).
• Program.cs converts to top-level statements, matching the article's snippets and removing a class that shared its name with its namespace.
• Two idiom lifts, both in code the article prints: ArgumentNullException.ThrowIfNull(name) in NodeSeparatorPolicy, and name[1..] in CamelCasePolicy. Both land in the article's code blocks in the same edit.
• Person.surName is left exactly as it is on purpose. The deliberately lower-cased property is the article's demonstration that the serializer copies member names verbatim.

Verification

SDK 10.0.302, runtime 10.0.10.

• dotnet build -c Release: 0 errors, 0 warnings.
• dotnet test -c Release: 7/7 passing (3 existing plus the 4 moved).
• dotnet run prints, in order:

{"GivenName":"Name1","surName":"Surname1"}
{"givenName":"Name1","surName":"Surname1"}
{"given/name":"Name1","sur/name":"Surname1"}
{"property_name":"value"}
{"PROPERTY_NAME":"value"}
{"property-name":"value"}
{"PROPERTY-NAME":"value"}

Every output string quoted in the article matches this run.