ci: use actions/checkout@v6 and actions/setup-node@v6

`setup-node@v6` enables dependencies caching.

Release notes:

- https://github.com/actions/checkout/blob/main/CHANGELOG.md
- https://github.com/actions/setup-node/releases/tag/v6.0.0

Author: darrachequesne

.github/workflows/ci.yml

@@ -49,10 +49,10 @@ jobs:
 
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
 
       - name: Use Node.js ${{ matrix.node-version }}
-        uses: actions/setup-node@v4
+        uses: actions/setup-node@v6
         with:
           node-version: ${{ matrix.node-version }}