Add merged pull requests for 0.6.{7,8} to CHANGELOG

[paulcadman]

This PR adds merged PR information to the CHANGELOG for the 0.6.7 and 0.6.8 releases.

For future reference:

I used the following vim regexes to convert from the GitHub release "changes" output to the format we use in our CHANGELOG:

s/*/-
s/https:\/\/github.com\/anoma\/juvix\/pull\/\(\d*\)/\[\\#\1](\0)
s/\(.*?\)by \(@.*?\) in \(\[.*\)/\1\3 \2
s/@\(\(\w\|-\)*\)$/([\1](https:\/\/github.com\/\1))