* Bump RAILS_VERSION
* Bundle
* rake update_versions
* rake changelog:header

Fix tests on Mail 2.7.1

It isn't allowed to execute `assert_enqueued_email_with` without block
See #33243

We might allow to do it in Rails 6.0. See #33258.

* Eager autoload mail gem when eager load is true

We had a production issue where our Sidekiq worker threads all became
deadlocked while autoloading a file within the mail gem, required via
ActionMailer, despite setting our Rails applicaiton to eager load.
`Mail.eager_autoload!` exists and works great, ActionMailer just doesn't
call it during eager loading. Adding it to the ActionMailer Railtie's
eager_load_namespaces takes care of calling `Mail.eager_autoload!`
during the `eager_load!` initializer.

* 'Mail' isn't defined yet, use before_eager_load instead

* Make sure mail is loaded

* Move eager load of Mail into ActionMailer.eager_load!

[Samuel Cochran + Rafael Mendonça França]

[ci skip]

README in 5-2-stable should point files and path to 5-2-stable instead of master branch.

[ci skip]

- And move the Active Job related section down. Otherwise it was
  appearing as if the options are available for the `delivery_job`
  setting.

Follow up of #31390.

Make clear that the files are not to be run for interpreters.

Fixes #23847.
Fixes #30690.
Closes #23878.

Reverts 4d96be1c
References #31026

This basically reverts cd9cc721

* Add missing credit
* Add backticks
* Fix indentation
* Remove trailing spaces

And some minor tweaks.