Replace Is.EqualTo(default) with Is.Default

[manfred-brands]

With new overloads to EqualTo in 4.3.0 we can no longer uniquely determine default.
But even in older cases, default would always mean default(object) which would be null.
This is likely not the intention of the caller when they use:

Assert.That(false, Is.EqualTo(default));`

As this compares a boolean with null which always fails:

Assert.That(true, Is.EqualTo(default))
  Expected: null
  But was:  True
Assert.That(false, Is.EqualTo(default))
  Expected: null
  But was:  False

Add a rule that warns about Is.EqualTo(default) and convert this to Is.Default for NUnit 4.
For NUnit 3 we could translate:

Assert.That<TActual>(actual, Is.EqualTo(default));

with:

Assert.That<TActual>(actual, Is.EqualTo(default(TActual));