[TASK] gitlab-ci nightly slack notification

Resolves: #93592
Releases: master, 10.4, 9.5
Change-Id: If851b9264d7d9e3b2a18df49d2c445f034aa5a80
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/68133


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>