Improve Slack announcement

Now that #9 is landed there's an improvement we can make regarding communicating the status of deployment in the Slack message.

Brainstorming a bit with @Gudahtt yesterday we had discussed updating the message once the deployment has run to better communicate the status. Since the message is currently static a developer could end up clicking it only to then realise somebody else has already approved the workflow run. Ideally the message should have some sort of status to indicate that it's waiting to be published (maybe like a yellow coloured avatar or something) or that the publish has already been run (update the avatar to green?).

Alternatively we could even delete the message entirely which might be even better and would cut down the noise in the Slack channel (which is always a good thing).

see https://api.slack.com/messaging/modifying and https://github.com/slackapi/slack-github-action#update-the-message