C#: Re-factor some of the existing tests.

[michaelnebel]

In this PR we

• Normalize the Models as Data expected output (this should give rise to fewer test expected test output updates when models are updated, as the tests no longer refers to the builtin id).
• Adding C# stubs for .NET 8 Runtime
• Convert existing query tests to use stubs instead of dll's.
• Fix compilation error and deprecation warning on an existing query.

Review on a commit by commit basis is recommended.