PHP 8.1: fix various deprecations/errors in newest version of PHP (#310)

* Test on PHP 8.1

* PHP 8.1: fix deprecated NULL param to glob()

* PHP 8.1: fix PHP error when passing NULL to rawurlencode()

* PHP 8.1: calling ctype_lower with FALSE is deprecated

* PHP 8.1: passing NULL to setAttribute() is deprecated

* PHP 8.1: passing NULL to str_replace() is an error

* PHP 8.1: fix error passing NULL to str_replace()

* PHP 8.1: fix return type deprecation with backwards compatible attribute

* Revert typo

library/HTMLPurifier/PropertyListIterator.php

@@ -29,6 +29,7 @@ class HTMLPurifier_PropertyListIterator extends FilterIterator
     /**
      * @return bool
      */
+    #[\ReturnTypeWillChange]
     public function accept()
     {
         $key = $this->getInnerIterator()->key();