build: Bump the harfbuzz req
We are using the hb metrics api in more places now, so just require 2.6.0 where it was introduced. That version is almost 2 years old now, so that is hopefully not too much of a burden.
We are using the hb metrics api in more places now, so just require 2.6.0 where it was introduced. That version is almost 2 years old now, so that is hopefully not too much of a burden.