Update deprecation.rails guides [ci skip]

Both `:gem_name` and `:deprecation_horizon` where added to the `deprecation.rails` event some time ago in Rails 5.2.0. See Changelog entry "Pass gem name and deprecation horizon to deprecation notifications.".

https://github.com/rails/rails/blob/5-2-stable/activesupport/CHANGELOG.md#rails-520-april-09-2018

This was added to be documented in the Rails Guides roughly a year ago, however, it was not back ported to previous Rails Guides versions (such as 6.1 here).

https://github.com/rails/rails/pull/47305/files