Implement interface generator adds `static` modifier for assignment operators

[AlekseyTs]

interface I1
{
    void operator ++();

    public void operator -=(int i);
 }

class C2 : I1
{
    public static void operator -=(int i)
    {
        throw new NotImplementedException();
    }

    public static void operator ++()
    {
        throw new NotImplementedException();
    }
}

class C3 : I1
{
    static void I1.operator -=(int i)
    {
        throw new NotImplementedException();
    }

    static void I1.operator ++()
    {
        throw new NotImplementedException();
    }
}

[CyrusNajmabadi]

Fixed with #78305