Drop Emacs 25.1 support

Test org-journal-carryover-delete-empty-journal-test condition:
    (ert-test-failed
     ((should
       (equal "No journal entry for this date."
	      (org-journal-read-or-display-entry ... ...)))
      :form
      (equal "No journal entry for this date." #<buffer 20190101>)
      :value nil :explanation
      (different-types "No journal entry for this date." #<buffer 20190101>)))
   FAILED   2/10  org-journal-carryover-delete-empty-journal-test

Probably a carryover problem, but not worth the hassle as Emacs 25.1 was
released around 8 years back.

.github/workflows/test.yml

@@ -17,7 +17,6 @@ jobs:
     strategy:
       matrix:
         emacs_version:
-          - 25.1
           - 26.1
           - 27.1
           - 28.1

org-journal.el

@@ -7,7 +7,7 @@
 
 ;; URL: http://github.com/bastibe/org-journal
 ;; Version: 2.2.0
-;; Package-Requires: ((emacs "25.1") (org "9.1"))
+;; Package-Requires: ((emacs "26.1") (org "9.1"))
 
 ;; Redistribution and use in source and binary forms, with or without
 ;; modification, are permitted provided that the following conditions are met: