Test Sensei and Sensei Pro on PHP 8.1

[donnapep]

• Add define( 'WP_DEBUG_DISPLAY', true ); in wp-config.php.
• Test all Sensei or Sensei Pro specific functionality on PHP 8.1.
• Fix any PHP notices / errors / warnings displayed in WP admin or on the frontend.

[renatho]

It's probably worth already check it in PHP 8.2. From this, I noticed we have many deprecation notices with PHP 8.2. Reproduced locally.

[donnapep]

It's probably worth already check it in PHP 8.2. From this, I noticed we have many deprecation notices with PHP 8.2. Reproduced locally.

@renatho There's a separate issue for PHP 8.2. 🙂