Update release error tracking is available in to 11.8

doc/user/project/operations/error_tracking.md

 # Error Tracking
 
-> [Introduced](https://gitlab.com/groups/gitlab-org/-/epics/169) in GitLab 11.7.
+> [Introduced](https://gitlab.com/groups/gitlab-org/-/epics/169) in GitLab 11.8.
 
 Error tracking allows developers to easily discover and view the errors that their application may be generating. By surfacing error information where the code is being developed, efficiency and awareness can be increased.