Make oxford commas now an error

doc/.linting/vale/styles/gitlab/OxfordComma.yml

doc/administration/auth/authentiq.md

doc/administration/auth/ldap-ee.md

doc/administration/auth/ldap.md

doc/administration/compliance.md

...stration/geo/disaster_recovery/background_verification.md

doc/administration/geo/disaster_recovery/planned_failover.md

doc/administration/geo/replication/datatypes.md

doc/administration/geo/replication/index.md

doc/administration/git_annex.md

doc/administration/incoming_email.md

doc/administration/index.md

...istration/monitoring/performance/grafana_configuration.md

doc/administration/restart_gitlab.md