Required library has code deprecation

It seems to me that mkalkbrenner/php-htmldiff-advanced is pretty much abandoned. I don't think the diff module should rely on it. Since it's GPL-2.0, the 2 classes could be brought into this module and the dependency changed to a recent version of caxy/php-htmldiff.

I think this should be elevated to Major since PHP 8.4 is supported by Drupal 11.

I guess on new installs it gets the new release so not Major. I think it could be a Minor to just ditch the extra dependency and bring in its classes into diff module.

It will remove the need for the hook_requirements, phpstan overrides, etc...