CI: test against Emacs 26, 27, 28 as well

Author: ruediger

.github/workflows/test.yml

@@ -7,7 +7,7 @@ jobs:
       fail-fast: false
       matrix:
         # Skipping some in between releases.
-        emacs: ["25.3", "29.1", "30.2"]
+        emacs: ["25.3", "26.3", "27.2", "28.2", "29.1", "30.2"]
     steps:
       - uses: actions/checkout@v6