Deprecation warning in PHP >= 8.1

[wiegertschouten]

Hi there,

Thanks for this nice plugin. However, under PHP 8.1 and higher, I get the following deprecation warning in a post list when I use a taxonomy filter:

Deprecated: Constant FILTER_SANITIZE_STRING is deprecated in /path/to/my/site/wp-content/plugins/admin-taxonomy-filter/inc/controller.php on line 53

This Stackoverflow answer might shed some light on how to fix this: https://stackoverflow.com/questions/69207368/constant-filter-sanitize-string-is-deprecated

If a PR is helpful, I can try and make one. Let me know.