CI: Update action versions

"Node.js 16 actions are deprecated. Please update the following actions
to use Node.js 20"

.github/workflows/publish.yml

@@ -13,10 +13,10 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     # Checks-out your repository under $GITHUB_WORKSPACE, so your job can access it
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
 
     - name: Cache RubyGems
-      uses: actions/cache@v3
+      uses: actions/cache@v4
       env:
         cache-name: rubygems
       with: