What would you like to be added:
I think it would be better to support slack group mentions.

Why is this needed:
If someone is assigned to and leaves the development team, it is difficult to change the PipeCD notification settings every time.

Currently pipeCD is supporting individual user mention, but the syntax for individual user mentions and group mentions is different in Slack.
refs: https://api.slack.com/reference/surfaces/formatting#mentioning-groups

As a workaround, if I put the format <!subteam^ID> into mentionedAccounts, it gets displayed as <@@subteam_name> in the Mention To section of the Slack message, with double @@ and even angle brackets <>