ci: bump checkout and setup-node to v5 (Node 20 deprecation)

GitHub is deprecating the Node 20 runtime for actions; checkout@v4 and
setup-node@v4 were being force-run on Node 24 with a warning. v5 targets
Node 24 natively. No workflow behavior change.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: ivandrenc

.github/workflows/ci.yml

@@ -16,9 +16,9 @@ jobs:
       matrix:
         node: [20, 22]
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v5
 
-      - uses: actions/setup-node@v4
+      - uses: actions/setup-node@v5
         with:
           node-version: ${{ matrix.node }}
           cache: npm