Add OTG details for CC7 end-of-life

docs/centos7/index.md

 # What is CERN CentOS 7 (CC7) ?
 
+!!! danger "CC7 is due to be deprecated within the CERN environment on 30.06.2024, see [OTG0145248](https://cern.service-now.com/service-portal?id=outage&n=OTG0145248)"
+
+
 CERN Community ENTerprise Operating System 7 is the upstream [CentOS 7](http://www.centos.org/), built to integrate into the CERN computing environment but it is not a site-specific product: all CERN site customizations are optional and can be deactivated for external users.
 
 ## Documentation