chore: Slack notifications for issue/PR open/close actions

[trentm]

I am proposing this as an alternative to the issue and PR Slack
notifications we currently get from https://github.com/integrations/slack
because of the following issues with that integration:

• It is over-wordy, resulting in notification noise masking any
  other conversation in the channel. This lowers the signal-to-noise
  ratio in the channel, which discourages meaningful conversation.
  • It includes some body content from issues/PRs. This may be a nice
    idea for a repo with commonly simple and short issue/PR
    descriptions. However, in general it is just spam. It can also get
    silly for PRs that have large <details> blocks, which the Slack
    rendering doesn't hide. E.g. deps: Bump updatecli version to v0.89.0 opbeans-node#304
  • It includes some issue metadata, sometimes some chat-ops buttons
    ("Comment", "Close").
  • It includes a variable-height summary of actions/checks on PRs.
    This sounds nice, but it results in dynamic-height content in the
    chat area that discourages ongoing conversation.
• In the wordiness the (small, gray) text with the repo name is hard
  to find. We have these notifications for 4+ repos, so it is pain to have
  to hunt for that.
• It @-names people listed as reviewers for PRs and makes a threads
  of every PR to show when it is merged. This sounds like a nice idea,
  but in practice means that the "Threads" section of Slack is spammed
  with useless unread threads for merged PRs that I've reviewed.
• Basically the Slack integration is trying to be a fancy full-on
  chat-ops thingy, and I don't want the noise. I want higher signal.
• It doesn't support ignoring some PR categories: specifically ignoring
  dependabot/renovate PRs. Currently we have those scheduled for Sunday.
  Every Monday morning is a multi-page scroll of useless notifications
  about regular dependabot maintenance work. It is very easy to lose
  meaningful conversation from Friday or over the weekend in that
  noise.

---

I propose to discuss this, try it out. Then if we prefer it, we stop the issue and PR notifications from the existing GH/Slack integration (we can keep its notifications for some other things like unfurling links to GH issues/comments/code-ranges). And then we do this for the other repos we care about.

FWIW, at least the ignoring of dependabot/renovate PR notifications has been frequently requested of the Slakc integration (since 2019), but not supported: integrations/slack#951
There was a PR to implement something that would work for that, but it was never accepted.

[trentm]

• It can also get
  silly for PRs that have large <details> blocks, which the Slack
  rendering doesn't hide. E.g.