ci: Checkout to v6

Author: jcs090218

.github/workflows/test.yml

@@ -41,7 +41,7 @@ jobs:
             emacs-version: 27.2
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - uses: jcs090218/setup-emacs@master
         with:
           version: ${{ matrix.emacs-version }}