Switch to 'Keep a Changelog' to maintain changelogs

[avivace]

See: https://keepachangelog.com/en/1.1.0/

[Rangi42]

This looks very similar to how we already structure our changelogs, with Added, Changed, and Removed headings, bullet-pointed changes, ISO-formatted dates, and (obviously) change descriptions written for the users, not just copy+pasting our Git commit messages. Unlike their example, we put deprecations/removals on top, but I think that's more in tune with their own advice: "If you do nothing else, list deprecations, removals, and any breaking changes in your changelog."

Can you explain what you/we should do to resolve this issue?