git_manager 2.0.23: admin tab icon and modern release files

• Updated: The Git tab of the top menu shows the git icon in the admin4 design (Icon=git in
  [Topmenu_gitmanager] of menu.ini.append). Older admin designs ignore the setting.
• Updated: composer.json requires PHP 8.0 and the Exponential legacy installer, names the
  extension directory for both installer keys, links the issues and the source, and lists the
  original author and 7x as maintainer.
• Updated: ezinfo.php and extension.xml state the release, the same copyright, the license and
  the website.
• Updated: The README is rewritten for new users: installation with Composer and by hand, every
  setting with an example, the dashboard's cards, the policy functions, the command line tools,
  upgrading, known limits and troubleshooting.