ci: upgrade GitHub Actions to Node.js 24-compatible versions

Update shared workflow action versions ahead of the June 2, 2026 deadline
when GitHub forces all actions from Node.js 20 to Node.js 24:

- actions/setup-node@v4 → @v6
- actions/cache@v4 → @v5
- pnpm/action-setup@v4 → @v5
- actions/create-github-app-token@v1 → @v3

Affects: plugin-ci.yaml, plugin-release.yaml, ci-health-check.yaml

Co-Authored-By: Paperclip <noreply@paperclip.ing>

.github/workflows/ci-health-check.yaml

@@ -14,7 +14,7 @@ jobs:
 
       - name: Generate GitHub App token
         id: app-token
-        uses: actions/create-github-app-token@v1
+        uses: actions/create-github-app-token@v3
         continue-on-error: true
         with:
           app-id: ${{ secrets.RELEASE_APP_ID }}