ci: bump checkout + setup-node to v5 (Node 24)

actions/checkout@v4 and setup-node@v4 run on the deprecated Node 20 runtime
(removed from runners 2026-09-16). v5 runs on Node 24 — kills the deprecation
annotation, no behavior change.

Author: raymondchins

.github/workflows/ci.yml

@@ -16,10 +16,10 @@ jobs:
         node-version: [18, 20, 22]
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@v5
 
       - name: Setup Node ${{ matrix.node-version }}
-        uses: actions/setup-node@v4
+        uses: actions/setup-node@v5
         with:
           node-version: ${{ matrix.node-version }}