Support Emacs 26.1 again

Since [1: 1ebac69] Emacs 26.1 effectively wasn't supported anymore and
[2: fa83e81] made that formal, but we should support that release at
least a while longer, which do by using the Compat package.

1: 2023-07-18 1ebac69
   package-recipe-validate-all: Report progress

2: 2023-12-24 fa83e81
   Fix CI build by requiring a newer Emacs

package-build.el

@@ -13,7 +13,7 @@
 ;; Keywords: maint tools
 
 ;; Package-Version: 4.0.0.50-git
-;; Package-Requires: ((emacs "27.1"))
+;; Package-Requires: ((emacs "26.1") (compat "27.1"))
 
 ;; SPDX-License-Identifier: GPL-3.0-or-later