Removing references to older GitLab releases

doc/administration/auth/ldap/google_secure_ldap.md

doc/administration/auth/ldap/index.md

doc/administration/auth/ldap/ldap_synchronization.md

doc/administration/auth/oidc.md

doc/administration/auth/smartcard.md

doc/administration/credentials_inventory.md

doc/administration/moderate_users.md

doc/administration/raketasks/ldap.md

doc/administration/settings/rate_limit_on_users_api.md

doc/api/access_requests.md

doc/api/appearance.md

doc/api/applications.md

doc/api/avatar.md

doc/api/group_access_tokens.md

doc/api/members.md

doc/api/plan_limits.md

doc/api/project_access_tokens.md

doc/api/settings.md

doc/api/users.md

doc/development/cascading_settings.md