Update for emacs 28

[izahn]

Emacs 28 was released today with a huge set of changes and improvements: https://github.com/emacs-mirror/emacs/blob/emacs-28.1/etc/NEWS

This is good news because it means we can remove some things that are now better supported by default. It also means that we have some work to do to remove obsolete packages and settings from our setup.

TODO:

• Try running this config with emacs 28 and note anything that doesn't work
• Review https://github.com/emacs-mirror/emacs/blob/emacs-28.1/etc/NEWS and identify new features that replace packages we use here
• Remove obsolete packages
• Update documentation, possibly require emacs 28 or newer