Make IntegerRangeType constructor private

[herndlm]

I was blindly following that comment (see diff), but tbh I did not fully understand it. Is it fine to do this or should it be rather deprecated via phpdoc?

Something should be done I think to avoid people using it instead of the static create function.

[ondrejmirtes]

I'm delighted! I totally missed this when bumping to 7.2+ :)

It's fine to make it private because currently for new IntegerRangeType(1, 5) we report this:

 ------ ----------------------------------------------------------------------------------------------
  Line   test.php
 ------ ----------------------------------------------------------------------------------------------
  3      Creating new PHPStan\Type\IntegerRangeType is not covered by backward compatibility promise.
         The class might change in a minor PHPStan version.

[ondrejmirtes]

Thank you.