Bump node from v18 to v21

Author: xt0rted

.github/workflows/ci.yml

@@ -25,7 +25,7 @@ jobs:
       - name: Install Node
         uses: actions/setup-node@v4.0.0
         with:
-          node-version: 18
+          node-version: 21
 
       - uses: xt0rted/markdownlint-problem-matcher@v2.0.0