Notify Slack channel about upcoming releases (#5197)

## 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)

.github/workflows/release-notify.yml

@@ -0,0 +1,29 @@
+name: Notify Slack channel about upcoming release
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.number }}
+  cancel-in-progress: true
+
+on:
+  pull_request:
+    branches:
+      - release
+    types:
+      # Default types that triggers a workflow:
+      # - https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#pull_request
+      - opened
+      - synchronize
+      - reopened
+      # Additional types that we want to handle:
+      - closed
+
+jobs:
+  notify:
+    runs-on: [ ubuntu-latest ]
+
+    steps:
+      - uses: neondatabase/dev-actions/release-pr-notify@main
+        with:
+          slack-token: ${{ secrets.SLACK_BOT_TOKEN }}
+          slack-channel-id: ${{ vars.SLACK_UPCOMING_RELEASE_CHANNEL_ID || 'C05QQ9J1BRC' }} # if not set, then `#test-release-notifications`
+          github-token: ${{ secrets.GITHUB_TOKEN }}