ci: bump checkout/setup-node to v6 (Node 24 runtime)

GitHub deprecated Node.js 20 for actions: actions/checkout@v4 and
actions/setup-node@v4 run on Node 20 and emit a deprecation warning.
Node 24 becomes the forced default on 2026-06-02 and Node 20 is removed
on 2026-09-16. Bump both to @v6 (Node 24). Config is unchanged —
node-version: 22 and registry-url are both supported in v6.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.github/workflows/release.yml

@@ -20,8 +20,8 @@ jobs:
   release:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
-      - uses: actions/setup-node@v4
+      - uses: actions/checkout@v6
+      - uses: actions/setup-node@v6
         with:
           node-version: 22
           registry-url: https://registry.npmjs.org