For EF Core, converting Enumerable.Count() to .Count may cause problems

It would be good to mention that using `Length/Count` property instead of `Enumerable.Count()` may cause issues when using it as part of Entity Framework query as it will change where the count is executed (client for properties, server for function call)

For example, in the following code, the query1 works while query2 throws `NullReferenceException`:

```CSharp
var query1 = Contracts
.AsNoTracking()
.Select(c => new {
Id = c.Id,
UsersCount = c.Users.Count()
});

var query2 = Contracts
.AsNoTracking()
.Select(c => new {
Id = c.Id,
UsersCount = c.Users.Count
});
```

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* ID: aa3a4825-9d3f-6ea4-e93c-82f34c1c1add
* Version Independent ID: d9c26240-07ee-e8f5-4c7c-e3ad3dbd9efd
* Content: [CA1829: Use Length/Count property instead of Enumerable.Count method (code analysis) - .NET](https://docs.microsoft.com/en-us/dotnet/fundamentals/code-analysis/quality-rules/ca1829)
* Content Source: [docs/fundamentals/code-analysis/quality-rules/ca1829.md](https://github.com/dotnet/docs/blob/main/docs/fundamentals/code-analysis/quality-rules/ca1829.md)
* Product: **dotnet-fundamentals**
* GitHub Login: @mavasani
* Microsoft Alias: **mavasani**