ci: use node lts in deploy docs workflow

Some library uses an operator which is not available in node 14 and therefore
deploy docs always fails. We host this documentation on gh-pages and link to it
in our `README.md` so this should be up to date.

.github/workflows/deploy-docs.yml

@@ -12,7 +12,7 @@ jobs:
       - name: Setup Node.js
         uses: actions/setup-node@v4
         with:
-          node-version: '14.x'
+          node-version: '20'
       # Pre-check to validate that versions match between package.json
       # and package-lock.json. Needs to run before npm install
       - name: Validate package.json and package-lock.json versions