Address Grafana deprecation warnings in 16.3 pre-install config check

With GitLab version 16.3, Grafana has been removed from Omnibus, and the associated gitlab.rb config sections are deprecated. The presence of any associated config lines will trigger the pre-install gitlab-ctl check-config to fail.

This needs to be resolved in GET 2.8.x for users by ensuring no Grafana related config lines are present in gitlab.rb when installing version 16.3+