Paid Memberships Pro - Advanced Levels Page Shortcode 1.1

* BUG FIX/ENHANCEMENT: Now using an unordered-list on comparison table mobile view for better spacing between items.
* BUG FIX: Fixing bug where comparison table mobile view was showing the incorrect comparison items.
* REFACTOR: Cleaned up some code and logic around showing the "Renew" button on the levels page.

* ENHANCEMENT: Improved support for the Custom Level Cost Text Add On and free levels. (@MaximilianoRicoTabo, @andrewlimaza)
* ENHANCEMENT: Added the ability to show the level's description within the compare table layout. (@dparker1005)
* ENHANCEMENT: Improved compatibility for PHP8+. (@JarrydLong)
* BUG FIX: Fixed an issue where the discount code attribute would apply to all shortcodes on a page (if there were more than one shortcode on a single page). (@MaximilianoRicoTabo)
* BUG FIX: Fixed an issue with compare table, when no `levels` attribute were supplied in the shortcode. (@andrewlimaza)
* REFACTOR: Removed duplicate "template" attribute from shortcode. (@andrewlimaza)
* REFACTOR: Moved to using the default get_option instead of pmpro_getOption functions. (@JarrydLong)