Fix artifact actions to use v4 instead of deprecated @master

Author: smessmer

.github/workflows/backend.yml

@@ -36,7 +36,7 @@ jobs:
         working-directory: backend
         run: npm run build
       - name: Store artifacts
-        uses: actions/upload-artifact@master
+        uses: actions/upload-artifact@v4
         with:
           name: sam-build
           path: backend/.aws-sam
@@ -56,7 +56,7 @@ jobs:
       - name: Setup AWS SAM
         uses: aws-actions/setup-sam@v2
       - name: Load artifacts
-        uses: actions/download-artifact@master
+        uses: actions/download-artifact@v4
         with:
           name: sam-build
           path: backend/.aws-sam