Replace deprecated slack notification in provider.yaml with new one

The Slack Notification listed in provider.yaml for slack has
been still using the deprecated version of it. Since the
provider verification of ours did not check neither notifications
nor warnings, it was not found before.

Found in the attempt of adding new verification in #33608 then
also detected by #33640 that improved existing verification.

airflow/providers/slack/provider.yaml

@@ -82,4 +82,4 @@ connection-types:
     connection-type: slackwebhook
 
 notifications:
-  - airflow.providers.slack.notifications.slack_notifier.SlackNotifier
+  - airflow.providers.slack.notifications.slack.SlackNotifier