Upgrade GitHub CI Node.js from 18 to 20

Summary:
Node 18 reached EOL in April 2025. The `selfsigned` package (transitive
dependency via webpack-dev-server/Docusaurus) was updated to v2.x which
depends on `peculiar/x509` requiring Node >= 20, causing `yarn install
--frozen-lockfile` to fail in CI.

Reviewed By: pbhandar2

Differential Revision: D99896960

fbshipit-source-id: bdca4ddcc7514e81abf02b29dec488424801a4a1

Author: AlnisM

.github/workflows/deploy-website.yml

@@ -27,7 +27,7 @@ jobs:
       - name: Setup Node.js
         uses: actions/setup-node@v4
         with:
-          node-version: 18
+          node-version: 20
           cache: yarn
           cache-dependency-path: website/yarn.lock