update actions/cache to v4

Author: ndorfin

.github/workflows/eleventy_build.yml

@@ -17,7 +17,7 @@ jobs:
           node-version: '16.10.0'
 
       - name: Cache Node.js modules
-        uses: actions/cache@v2
+        uses: actions/cache@v4
         with:
           # npm cache files are stored in `~/.npm` on Linux/macOS
           path: ~/.npm