Don't copy CHANGELOG.md on Release

We need to avoid storing release notes in two places. If manual edits are needed (which is somewhat common), it is a huge pain and error prone when the notes are in multiple places. At the moment we utilize the CHANGELOG feature of GitLab to update the CHANGELOG.md file and copy the most recent changes into the GitLab Release feature's release notes on release.

If users forget to add the changelog: changed (or similar) trailer users will likely need to update the release notes manually, and this is onerous if it's two places. Coming from GitHub I didn't have this issue because they don't steer you to use a CHANGELOG file in GitHub. I'd prefer to just keep all release notes in the release feature, but GitLab will indefinitely prompt you to add a CHANGELOG file (on the right side of your project page) if you don't have one, plus it appears to be the standard way in GitLab to generate a set of changes in the release so we can't easily not use it. Maybe we just include a link (with page section anchor) to CHANGELOG.md when browsing releases from the releases page.

GitLab appears to suggest that release notes are high level summaries for end-users, whereas CHANGELOG is more fine grained and for developers. This probably makes sense in some projects, but for us this is mostly an unnecessary extra burden to make that distinction.

Aside: We still need to figure out how Merge Requests are handled too. They need to be included in the release notes as well.