ci: upgrade to Node 20 for coverage support

- Node 18 lacks node:inspector/promises required by @vitest/coverage-v8
- Node 20 provides full compatibility

Author: jonmatum

.github/workflows/ci.yml

@@ -17,7 +17,7 @@ jobs:
 
       - uses: actions/setup-node@v4
         with:
-          node-version: '18'
+          node-version: '20'
           cache: 'npm'
 
       - run: npm ci