fix: handle string nodejs signals (#55606)

## Fixing a bug

### What?

On these versions, SIGINT signals are `string`, exit callback recieves code `number`

### How?

We use code argument to quit process for `event`, and exit with code `0` for `SIGINT` and `SIGTERM` signals 

Fixes #55605


Co-authored-by: Jiachi Liu <4800338+huozhi@users.noreply.github.com>

packages/next/src/server/lib/start-server.ts

@@ -282,9 +282,10 @@ export async function startServer({
           // This is the render worker, we keep the process alive
           console.error(err)
         }
-        process.on('exit', cleanup)
-        process.on('SIGINT', cleanup)
-        process.on('SIGTERM', cleanup)
+        process.on('exit', (code) => cleanup(code))
+        // callback value is signal string, exit with 0
+        process.on('SIGINT', () => cleanup(0))
+        process.on('SIGTERM', () => cleanup(0))
         process.on('uncaughtException', exception)
         process.on('unhandledRejection', exception)