System.Linq doc snippets (\dotnet-api-docs\snippets\csharp\System.Linq

1. the [LINQ docs C# samples](https://github.com/dotnet/dotnet-api-docs/tree/main/snippets/csharp/System.Linq) consist of 7 projects to showcase **IEnumerable** and **IQueryable**
unfortunately much code unnecessarily invokes **AsQueryable()** despite simpler **IEnumerable** method implementations existing
[thus such guidance is poor as such **AsQueryable()** overhead can be avoided]
2. should update to useful modern C# v12 semantics [Primary constructors, Collection expressions, Span, is, etc]
3. calls being made to default parameterless ctor then assigning individual properties, rather than using parameterised ctors explicitly [or better yet primary constructor]
4. current analyzers also disclose numerous Warnings/Messages, emphasising poor guidance for today's coding
5. many cases of wastefully using Any(),Count(),LongCount etc when optimal coding available (e.g. Count, Length for Collections).
6. spurious ToString() call in [OrderByIComparer()](https://github.com/dotnet/dotnet-api-docs/blob/main/snippets/csharp/System.Linq/Enumerable/AggregateTSource/enumerable.cs#L1828)

I will shortly submit a PR to explain problems & my suggestions.