Fix issue with incomplete property overrides in C# sourcegen

[paulloz]

This PR fixes a crash in several generators, preventing compilation through CS8785.

To filter read-only properties, we currently check if the property is read-only (duh), or is its set method is init-only. In reality, we cannot take for granted the fact a set method is available to check, even when the property is not read-only. This is because C# allows the following.

class Foo
{
    // Define a readable, and writable property as virtual
    public virtual string MyString
    {
        get;
        set;
    }
}

class Bar : Foo
{
    // Only override the get accessor
    public override string MyString
    {
        get { return "Gotcha"; }
    }
}

Since we assume we have a set method to check, we're raising an NRE when checking for the init-only flag. To fix this, we're now recursing on the overridden property if it makes sense.

Cheers ✌️

• Bugsquad edit, closes: .NET 6 C# - ScriptPropertiesGenerator fail with overridden initialized property on master #71102