Alerting: Fix Slack push notifications (#52391)

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.

pkg/tests/api/alerting/api_notification_channel_test.go

@@ -2099,6 +2099,7 @@ var expNonEmailNotifications = map[string][]string{
 	"slack_recv1/slack_test_without_token": {
 		`{
 		  "channel": "#test-channel",
+          "text": "Integration Test [FIRING:1] SlackAlert1 (default)",
 		  "username": "Integration Test",
 		  "icon_emoji": "🚀",
 		  "icon_url": "https://awesomeemoji.com/rocket",
@@ -2128,6 +2129,7 @@ var expNonEmailNotifications = map[string][]string{
 	"slack_recvX/slack_testX": {
 		`{
 		  "channel": "#test-channel",
+          "text": "[FIRING:1] SlackAlert2 (default)",
 		  "username": "Integration Test",
 		  "attachments": [
 			{