Create and review 2.1 API changelog and CHANGELOG.md

[sffc]

We need to compile the changelog so that we can formally approve the 2.1 release. I want to improve upon what we've done in the past: I would like this to not be rushed, meaning at least 7-10 days between an API-complete draft changelog and release date. Also new this time is the semver-check API change report, which should be included as part of the changelog to be approved by the Technical Committee.