Bump actions/checkout to v5 (Node.js 24)

Node.js 20 actions are deprecated and will be forced to Node.js 24
after June 2nd, 2026.

Author: bbatsov

.github/workflows/ci.yml

@@ -22,7 +22,7 @@ jobs:
             emacs-version: '29.4'
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v5
 
       - uses: jcs090218/setup-emacs@master
         with: