How do I transfer text messages from old iPhone to new iPhone?

Restore the iPhone 8+ from the iPhone 6S backup. See the "Here’s what iCloud Backup includes" section in:

What does iCloud back up? - Apple Support

which says iMessage, text (SMS), and MMS messages are included.

Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

There is also a new "Quick Start" option with iOS 11 that I am not familiar with:

Transfer content from your previous iOS device to your new iPhone, iPad, or iPod touch - Apple Support