Bump setup-node to 20

VSCE has this as lower bound now.

Author: sim642

.github/workflows/build.yml

@@ -22,10 +22,10 @@ jobs:
           distribution: 'adopt'
           cache: maven
 
-      - name: Set up Node.js 18
+      - name: Set up Node.js 20
         uses: actions/setup-node@v3
         with:
-          node-version: '18.x' # TODO: what version do we want?
+          node-version: '20.x' # TODO: what version do we want?
           # cache: npm # TODO: requires package-lock.json
 
       - name: Build with Maven