Update to the latest stable release.

The main changes since from version 20.07.0 are:

• A few bug fixes;
• More work on a modern look-and-feel;
• Updated the default MathJAX to version 3;
• A massive performance update;
• stringdisp support and a menu item for it;
• Many auto-subscripting improvements;
• An editor for maxima's environment variables;
• Repaired the non-case-sensitive "Replace All" functionality;
• More efficient and accurate calculation of worksheet object sizes;
• diff cells can now be broken into lines;
• Reduced the memory footprint of big equations;
• A better 2D ASCII Art mode;
• Better High-DPI support;
• If it is configurable it now might have a right-click menu;
• Reduced the memory footprint of cells used only when line breaking is performed;
• Subscripts created by derivabbrev are now correctly copied as "diff" command;
• Added buttons that reset the configuration;
• A few menu items now are checkboxes set and cleared by maxima;
• wxMaxima forgot to release the communication port it used for talking to maxima;
• Improved performance of receiving data from maxima;
• The font sizes are stored in configuration with full precision, without rounding to integer;
• Updated translations:
  • Chinese translation of the manual;
  • Italian translation;
  • Ukrainian translation;
  • Russian translation;
  • wxMaxima now will work even if no English locale is installed.