Error on legacy runtime config under app dir (#48774)
When you're using `config` object with `runtime` we'll show a warning and recomment you to move to `export const runtime = ...` for runtime property as it's documented as recommended. This PR adds a error for `next build` to avoid logs are truncated during multi-workers build and you're aware of the config which doesn't take effect with seeing the error
Follow up for #48630
link [NEXT-426](https://linear.app/vercel/issue/NEXT-426)
