[v9.0.x] Alerting: Fix Slack push notifications (#52476)

This commit fixes push notifications for Slack which used to show "This content cannot be displayed". The text field is shown in both the message and the push notification.

(cherry picked from commit 094dfd0d4d)

Co-authored-by: George Robinson <george.robinson@grafana.com>

pkg/tests/api/alerting/api_notification_channel_test.go

@@ -2118,6 +2118,7 @@ var expNonEmailNotifications = map[string][]string{
 	"slack_recv1/slack_test_without_token": {
 		`{
 		  "channel": "#test-channel",
+          "text": "Integration Test [FIRING:1] SlackAlert1 ",
 		  "username": "Integration Test",
 		  "icon_emoji": "🚀",
 		  "icon_url": "https://awesomeemoji.com/rocket",
@@ -2147,6 +2148,7 @@ var expNonEmailNotifications = map[string][]string{
 	"slack_recvX/slack_testX": {
 		`{
 		  "channel": "#test-channel",
+          "text": "[FIRING:1] SlackAlert2 ",
 		  "username": "Integration Test",
 		  "attachments": [
 			{