Bump CI actions and Node to versions requested by reviewer

actions/checkout and actions/setup-node both updated to v6; node-version bumped from 20 to 24 per @tvdijen's inline review comments on PR simplesamlphp#108.

Author: kellenmurphy

.github/workflows/test.yml

@@ -13,11 +13,11 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
-      - uses: actions/setup-node@v4
+      - uses: actions/setup-node@v6
         with:
-          node-version: 20
+          node-version: 24
 
       - name: Install dependencies
         run: npm ci