Notify devs when their change lands on the latest evergreen track

[sfreudenthaler]

Summary

When the release pipeline moves the floating latest Docker tag (the promote-latest job in cicd_6-release.yml), notify the GitHub users whose commits are newly included via Slack, telling them their change is live on latest and when the evergreen-tracks reconciler will roll it out to latest-track customer environments.

Why

Developers currently have no visibility into when their merged change actually reaches a running customer environment. The evergreen-tracks reconciler already runs a predictable weekday cron for the latest track (kubernetes/cluster/*/evergreen-tracks-reconciler/cronjob-latest.yaml, 10:00 AM ET onward per cluster) — we can compute the next scheduled rollout and tell the author directly.

Approach

• New reusable workflow cicd_comp_notify-latest-promotion.yml, called from cicd_6-release.yml right after promote-latest succeeds.
• Diffs commits between the current release tag and the previous one (via GitHub compare + commits/pulls API) to find PR authors.
• Resolves GitHub logins to Slack users via the existing utility_slack-channel-resolver.yml.
• Looks up which environments are currently subscribed to the latest track via the dotUsage GET /environments API (tenant/customer names), with a generic fallback if that lookup fails.
• Computes the next weekday 10:00 AM America/New_York rollout window and includes it in the Slack message.

Out of scope

• Per-cluster exact times (message references the earliest/reference window; other regions follow later the same day).
• Live Google-Sheet-based subscription source (dotUsage already mirrors this).

[github-actions]

PRs linked to this issue

• Notify devs via Slack when their change hits the latest track by @sfreudenthaler