If you go here: https://fiber.google.com/newcities/ and hover over the main cities it gives you a breakdown.

Carrboro, Cary, Chapel Hill, Durham, Garner, Morrisville, Raleigh

It has it in the hero picture at the top here: https://fiber.google.com/cities/raleighdurham/home/ as well.

From a live stream of the announcement it included Carrboro, Cary, Chapel Hill, Durham, Raleigh and Morrisville.

Now just to find a written source...