Bump node in github actions.

Brown paper bag: the environment.yml file is only for local development. GA does not use it.

.github/workflows/deploy.yml

@@ -33,7 +33,7 @@ jobs:
         uses: actions/configure-pages@v3
       - uses: actions/setup-node@v4
         with:
-          node-version: 18.x
+          node-version: 20.x
       - name: Install MyST Markdown
         run: npm install -g mystmd
       - name: Build HTML Assets