Is your feature request related to a problem? Please describe.
Currently, our release notes get posted to the releases channel on Mattermost whenever we do a release, i.e. deploy from the main branch to either beta or prod. We don't really every deploy from main to prod, though; we instead promote the builds from beta to prod, and that doesn't send release notes to Mattermost.

This used to not be a problem when we had rolling releases because everybody knew that whatever was being released to beta would released to prod a week later (or if something failed, two weeks later). Without the rolling releases, non-developers do not have an easy way of seeing what has been released to production recently. We should change that.

Describe the solution you'd like
Ideally, we change the promotion workflow to also update Mattermost.

Describe alternatives you've considered
We could manually post on Mattermost, which seems annoying and will probably get forgotten. Or we could do deployments from the main branch to production, but then we might deploy something that hasn't been on beta yet.

Additional context
This came up because the Social and Service teams wanted to know what has gone live recently. They shouldn't have to ask for each issue :)