avoid ?? in setupProxy.js

It worked fine on my laptop with node.js v14.15.4, but not on my
new workstation with node.js v12.18.2 (as comes with Ubuntu 20.10).
It would print "Unexpected token '?'" without any clue what file the
error is in. I'm trying to fix that here:
https://github.com/facebook/create-react-app/pull/10652

ui/src/setupProxy.js

@@ -10,7 +10,7 @@ module.exports = (app) => {
   app.use(
     "/api",
     createProxyMiddleware({
-      target: process.env.PROXY_TARGET ?? "http://localhost:8080/",
+      target: process.env.PROXY_TARGET || "http://localhost:8080/",
       ws: true,
       changeOrigin: true,