ci: install Node 22 for the frontend-tests job

Etherpad core bumped its engines.node requirement past 20.x; the
default GitHub Actions ubuntu-latest runner ships Node 20, so
bin/installDeps.sh now aborts with "Your nodejs version 20.20 is
too old. nodejs 22.0.x or higher is required" before any plugin
spec gets a chance to run. Add an explicit actions/setup-node@v4
step pinned to Node 22.

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

Author: JohnMcLear

.github/workflows/frontend-tests.yml

@@ -16,6 +16,10 @@ jobs:
         with:
           repository: ether/etherpad-lite
           path: etherpad-lite
+      - uses: actions/setup-node@v4
+        name: Install Node.js
+        with:
+          node-version: 22
       - uses: pnpm/action-setup@v6
         name: Install pnpm
         with: