Use node 18 for explorer in docs.yml

Author: jpraynaud

.github/workflows/docs.yml

@@ -92,7 +92,7 @@ jobs:
       - name: Setup Node.js
         uses: actions/setup-node@v3
         with:
-          node-version: 16
+          node-version: 18
           cache: 'yarn'
           cache-dependency-path: mithril-explorer/yarn.lock