False positive - C# IEquatable: Equals(object) not implemented alert when Equals(object?) is implemented

**Description of the false positive**

CodeQL will complain about a missing override for `Equals(object obj)` not existing, if the Equals method is implemented within a nullable object as parameter. Using `Equals(object? obj)` is valid.

Two different alerts appeared on a PR.

**Code sample**

Sadly, working in closed source, but this sample should do. I do not know if this triggers the false positive in practice, as I can't push it to test, but it should give you a good example. I also added a small a bit of code to show that Equals(object? obj) is valid and working when using `Equals()`.

```csharp
using System;
using System.Linq;

#nullable enable
public class Program
{
public class MyClass : IEquatable
{
private object property;
public MyClass()
{
property = 1;
}

public bool Equals(MyClass? other)
{
Console.WriteLine("MyClass? called");
if (ReferenceEquals(null, other))
{
return true;
}

return Equals(property, other.property);
}

public override bool Equals(object? obj)
{
Console.WriteLine("object? called");
if (ReferenceEquals(null, obj))
{
return false;
}

if (ReferenceEquals(this, obj))
{
return true;
}

if (obj.GetType() != GetType())
{
return false;
}

return Equals((MyClass)obj);
}

public override int GetHashCode()
{
return HashCode.Combine(property);
}
}

public static void Main()
{
var a = new MyClass();
var b = new MyClass();
Console.WriteLine(a.Equals(b)); // Equals(MyClass? other)
Console.WriteLine(Equals(a, b)); // Equals(object? obj)
Console.WriteLine(IEquatable.Equals(a, b)); // Equals(object? obj)
}
}
```
Check 1 (failure check)

The CodeQl error message goes as follows for the `override bool Equals(object? obj)` line:
```Class 'MyClass' does not implement Equals(object), but it implements IEquatable.Equals```
The `IEquatable.Equals` points to `bool Equals(MyClass? other)`.

Error 2 (alert):
The warning goes for the same function `override bool Equals(object? obj)`
```The declaring(MyClass) type of this 'Equals(Object)' method does not override 'Equals(object)'.```

**Source for alerts**
https://github.com/github/codeql/blob/main/csharp/ql/src/API%20Abuse/ClassDoesNotImplementEquals.ql

https://github.com/github/codeql/blob/28b6aa8616a393ebb45186e3ba4df004a0f3ef4e/csharp/ql/src/API%20Abuse/IncorrectEqualsSignature.ql