ci: move to checkout/setup-node v5

v4 targets Node 20, which GitHub runners now force onto Node 24 with a
deprecation warning on every run.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: garethx

.github/workflows/ci.yml

@@ -9,8 +9,8 @@ jobs:
   test:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
-      - uses: actions/setup-node@v4
+      - uses: actions/checkout@v5
+      - uses: actions/setup-node@v5
         with:
           node-version: 22
           cache: npm