Remove mention from aler group main slack message (#4637)

# What this PR does
Removes @mention from Alert Slack message
Fixes #187
## Which issue(s) this PR closes

Closes [issue link here]

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## Checklist

- [ ] Unit, integration, and e2e (if applicable) tests updated
- [ ] Documentation added (or `pr:no public docs` PR label added if not
required)
- [ ] Added the relevant release notes label (see labels prefixed w/
`release:`). These labels dictate how your PR will
    show up in the autogenerated release notes.

engine/apps/alerts/incident_appearance/renderers/slack_renderer.py

@@ -135,15 +135,15 @@ def render_alert_group_attachments(self):
         if self.alert_group.resolved:
             resolve_attachment = {
                 "fallback": "Resolved...",
-                "text": self.alert_group.get_resolve_text(mention_user=True),
+                "text": self.alert_group.get_resolve_text(mention_user=False),
                 "callback_id": "alert",
             }
             attachments.append(resolve_attachment)
         else:
             if self.alert_group.acknowledged:
                 ack_attachment = {
                     "fallback": "Acknowledged...",
-                    "text": self.alert_group.get_acknowledge_text(mention_user=True),
+                    "text": self.alert_group.get_acknowledge_text(mention_user=False),
                     "callback_id": "alert",
                 }
                 attachments.append(ack_attachment)