ci: bump checkout@v7 + setup-node@v6 off Node-20-deprecated pins

actions/checkout@v4 and actions/setup-node@v4 run on the deprecated Node 20
runtime (GitHub forces Node 24 with a warning). Bump to the latest majors,
both Node-24 native.

Changelog-checked: checkout v7 only changes fork-checkout behavior for
pull_request_target/workflow_run (this workflow uses pull_request + push —
unaffected); setup-node v6 limits auto-caching to npm (cache: npm already set
explicitly — unaffected).

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

Author: jsolly

.github/workflows/ci.yml

@@ -14,8 +14,8 @@ jobs:
     name: ci
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
-      - uses: actions/setup-node@v4
+      - uses: actions/checkout@v7
+      - uses: actions/setup-node@v6
         with:
           node-version-file: .nvmrc
           cache: npm