Upgrade setup-node to version 4 in the GitHub Action workflows

This major version mainly involves not using Node.js 16 internally
anymore, which will be end of life on September 11th. This prevents the
workflows from using an unsupported version of Node.js as well as
deprecation warnings getting printed in the workflow logs.

For more information please refer to
https://github.com/actions/setup-node/releases/tag/v4.0.0 and
actions/setup-node#850.

.github/workflows/ci.yml

@@ -15,7 +15,7 @@ jobs:
           fetch-depth: 0
 
       - name: Use Node.js 18 LTS
-        uses: actions/setup-node@v3
+        uses: actions/setup-node@v4
         with:
           node-version: '18'
 

.github/workflows/publish_website.yml

@@ -18,7 +18,7 @@ jobs:
           fetch-depth: 0
 
       - name: Use Node.js 18 LTS
-        uses: actions/setup-node@v3
+        uses: actions/setup-node@v4
         with:
           node-version: '18'