Notify Slack channel about upcoming releases

[bayandin]

Problem

If 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 if not configured)
• On each PR update, the workflow updates the list of commits in the message (it doesn't send additional messages)

Checklist before requesting a review

• I have performed a self-review of my code.
• If it is a core feature, I have added thorough tests.
• Do we need to implement analytics? if so did you add the relevant metrics to the dashboard?
• If this PR requires public announcement, mark it with /release-notes label and add several sentences in this section.

Checklist before merging

• Do not forget to reformat commit message to not include the above checklist

[github-actions]

1624 tests run: 1551 passed, 0 failed, 73 skipped (full report)

---

Code coverage full report

• functions: 53.3% (7452 of 13986 functions)
• lines: 81.5% (44036 of 54050 lines)

---

_{The comment gets automatically updated with the latest test results
ae56566 at 2023-09-06T13:49:50.184Z :recycle:}

[kelvich]

Quite a lot of code : ) Thank you!

[bayandin]

I've moved the biggest part of the code to a separate action.

Now it'll be easier to reuse (see https://github.com/neondatabase/cloud/pull/6772)