Deprecates Protected Paths setting

Protected paths throttle will be included on GitLab 12.4, see gitlab#29952 (closed) for more details. As part of this we are moving where protected paths can be configured.

This setting configuration is now available through the GitLab Admin UI, which is now the recommended place to configure the setting. Support for the setting in gitlab.rb is now deprecated, and will be removed in GitLab 13.0

See the migration guide for how to switch to using the GitLab Admin UI: https://docs.gitlab.com/ee/user/admin_area/settings/protected_paths.html#migrate-settings-from-gitlab-123-and-earlier

Related to #4688 (closed)