Upgrade HTMLPurifier (to v4.19.0?)

The (currently) latest version of FlexForm ships with HTMLPurifier v4.13.0 (2020), which is beginning to show its age. Because that version uses dynamic properties and PHP 8.2 is throwing warnings whenever they get used, it is probably time to upgrade the library. There are at least two classes in the older version of HTMLPurifier that depend on dynamic properties, i.e. Lexer and NameSync.

The question remains which version we need to upgrade to :

• The latest version listed on http://htmlpurifier.org/ is v4.15.0 (2022), which is said to contain various fixes for PHP 8, but the website does not appear to have been updated in a while as several links have gone dead.
• There is https://github.com/ezyang/htmlpurifier, which is actively maintained and is currently at v4.19.0. The maintainer admits he hasn't had time or occasion to update the website - https://github.com/ezyang/htmlpurifier/issues/400 - so there's that.

I don't know if upgrading directly to v4.19.0 is too steep a curve so I'll conveniently leave that up to you, @charlot