Update actions/checkout action to v4

.github/workflows/render-md-mermaid.yml

@@ -8,7 +8,7 @@ jobs:
 
     steps:
       - name: Checkout code
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
 
       - name: Render images for every Mermaid diagram in all Markdown files (*.md) in the repo
         uses: nielsvaneck/render-md-mermaid@v2