Update to action/checkout@4 (#23)

Author: johannes-mueller

.github/workflows/test.yml

@@ -30,7 +30,7 @@ jobs:
           - emacs_version: 'release-snapshot'
             allow_failure: true
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - uses: purcell/setup-emacs@master
         with:
           version: ${{ matrix.emacs_version }}