Insert the changelog content into the Github Release Info

[blimmer]

Currently, we link out to the changelog file in the github release for new versions:

atomic_cache/ci-helpers/gem_helper

Line 104 in 19eebb5

body: "See [changelog](https://github.com/Ibotta/atomic_cache/blob/#{tag}/CHANGELOG.md)",

However, it would be nice to be able to insert the relevant content so people don't have to jump over to the CHANGELOG file.

I think the way you'd want to do this is with a markdown AST, select out the last release until the next major header and use that as the content.