Bump the minimum Emacs version to 26.3! 🎉🤞

All the code working around issues in Emacs 24 and Emacs 25 has been
removed at this point! (or at least, as much of it as I could find)

I've tested against Emacs 28 and it's working fine. I need many more
automated tests to ensure that behaviour does not break across
multiple Emacs versions / operating systems, but that is a topic for
another day.

Closes: #26

lisp/pdf-tools.el

@@ -8,7 +8,7 @@
 ;; Keywords: files, multimedia
 ;; Package: pdf-tools
 ;; Version: 1.0.0snapshot
-;; Package-Requires: ((emacs "24.3") (nadvice "0.3") (tablist "1.0") (let-alist "1.0.4"))
+;; Package-Requires: ((emacs "26.3") (tablist "1.0") (let-alist "1.0.4"))
 
 ;; This program is free software; you can redistribute it and/or modify
 ;; it under the terms of the GNU General Public License as published by