fix(templates): pass PORT into next in custom servers (#6807)

Closes https://github.com/payloadcms/payload/issues/4800

- [x] Chore (non-breaking change which does not add functionality)

examples/custom-server/src/server.ts

@@ -38,6 +38,7 @@ const start = async (): Promise<void> => {
 
   const nextApp = next({
     dev: process.env.NODE_ENV !== 'production',
+    port: PORT,
   })
 
   const nextHandler = nextApp.getRequestHandler()