[2.4.0] Checkmk Werk 17177 created: Slack/Mattermost: Updated notification message

[//]: # (werk v2) # Slack/Mattermost: Updated notification message key | value ---------- | --- date | 2024-08-13T08:36:24+00:00 version | 2.4.0b1 class | feature edition | cre component | notifications level | 1 compatible | yes The Slack notification plugin now uses Block Kit, resulting in differently rendered notifications. Some of these formatting changes also apply to Mattermost notifications.