`Cookie.GetHashCode()` does not ignore leading dot in Domain

[antonfirsov]

There was an overlook in #64038: we switched behavior to ignore leading dot in Cookie.Domain for equality comparison, but it still affects Cookie.GetHashCode() calculation.

Cookie a = new("a", "aa") { Domain = "wow" };
Cookie b = new("a", "aa") { Domain = ".wow" };
Console.WriteLine(a.Equals(b)); // True
Console.WriteLine(a.GetHashCode() == b.GetHashCode()); // False :(

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