fix: bump minimum Node.js version to 16.8.0 (#48957)
We bumped `undici` fetch which has a minimum version of 16.8.0 so we need to make sure `next` and `create-next-app` also have the same minimum version.
Since 14.x reaches End-of-Life on [2023-04-30](https://github.com/nodejs/Release), we can drop support for 14 in the next release.
See also:
- Related to #48870
- Related #48941
