Fix CI build by requiring a newer Emacs

In a commit made by @tarsius a few months ago, this code started using
`dolist-with-progress-reporter`, which wasn't added until Emacs
27.1. The CI build has been broken since then on Emacs 26.x.

I guess it's acceptable to start requiring Emacs 27 to fix this.

package-build.el

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