## Problem
When the next release is coming, we want to let everyone know about it by
posting a message to the Slack channel with a list of commits.
## Summary of changes
- `.github/workflows/release-notify.yml` is added
- the workflow sends a message to
`vars.SLACK_UPCOMING_RELEASE_CHANNEL_ID` (or
[#test-release-notifications](https://neondb.slack.com/archives/C05QQ9J1BRC)
if not configured)
- On each PR update, the workflow updates the list of commits in the
message (it doesn't send additional messages)
