Update deep link URL for Slack messages (#4317)

# What this PR does

It updates the slack deep link url to respect the [correct
format](https://api.slack.com/reference/deep-linking#app_or_bot)
requested [here](https://github.com/grafana/oncall/issues/4122)

## Checklist

- [x] Unit, integration, and e2e (if applicable) tests updated
- [x] Documentation added (or `pr:no public docs` PR label added if not
required)
- [x] 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.

---------

Co-authored-by: Ildar Iskhakov <Ildar.iskhakov@grafana.com>

engine/apps/slack/models/slack_message.py

@@ -98,7 +98,7 @@ class SlackMessage(models.Model):
 
     @property
     def deep_link(self) -> str:
-        return f"slack://channel?team={self.slack_team_identity.slack_id}&id={self.channel_id}&message={self.slack_id}"
+        return f"https://slack.com/app_redirect?channel={self.channel_id}&team={self.slack_team_identity.slack_id}&message={self.slack_id}"
 
     def send_slack_notification(self, user, alert_group, notification_policy):
         from apps.base.models import UserNotificationPolicyLogRecord

engine/apps/slack/tests/test_slack_message.py

@@ -56,5 +56,8 @@ def test_slack_message_deep_link(
     slack_channel = make_slack_channel(slack_team_identity)
     slack_message = make_slack_message(alert_group=alert_group, channel_id=slack_channel.slack_id)
 
-    expected = f"slack://channel?team={slack_team_identity.slack_id}&id={slack_channel.slack_id}&message={slack_message.slack_id}"
+    expected = (
+        f"https://slack.com/app_redirect?channel={slack_channel.slack_id}"
+        f"&team={slack_team_identity.slack_id}&message={slack_message.slack_id}"
+    )
     assert slack_message.deep_link == expected