Rails 5 added the `preparable` argument to `select_all`, which was added
to help prepared statement caching via
https://github.com/rails/rails/commit/2a56b2d90d4fed8548e3a1e7a7b206454858c872.

This argument is not needed by the load balancer since prepared
statements are disabled, but it needs to be present or a method
signature error is thrown.

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/8692