fix(ci): update integrity workflow to Node 20 for yargs@18 compatibility

yargs@18.0.0 requires Node.js >=20.19.0 but the integrity-check
workflow was using Node 18, causing `yarn install` to fail with an
engine incompatibility error. Updated the workflow to Node 20 (matching
volta config and all other workflows) and aligned the engines field in
package.json accordingly.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: frostebite

.github/workflows/integrity-check.yml

@@ -24,7 +24,7 @@ jobs:
       - uses: actions/checkout@v4
       - uses: actions/setup-node@v4
         with:
-          node-version: '18'
+          node-version: '20'
       - run: yarn
       - run: yarn lint
       - run: yarn test:ci --coverage

package.json

@@ -45,7 +45,7 @@
     "test-i-k8s": "cross-env orchestratorTests=true providerStrategy=k8s yarn test -i -t \"orchestrator\""
   },
   "engines": {
-    "node": ">=18.x"
+    "node": ">=20.x"
   },
   "dependencies": {
     "@actions/cache": "^4.0.0",