How do I transfer text messages from old iPhone to new iPhone?
How do I transfer text messages from iPhone 6S to new iPhone 8+
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: