This reverts commit 89b4612f.

Add `--skip-action-mailbox` option to `rails new`

Add missing keys to ActiveSupport#instrumentation guide [skip ci]

Only define attribute methods from schema cache

Don't need to specify the version because Bundler 2.0 released.
Ref: https://bundler.io/blog/2019/01/03/announcing-bundler-2.html

fix activerecord reaper_test

 Remove mention about `Test::Unit::TestCase` [ci skip]

See https://github.com/rails/rails/blob/master/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb#L625.

Restore an ability that class level `update` without giving ids

Fix examples in ActiveSupport::LogSubscriber docs

We've switched from `Test::Unit::TestCase` to `ActiveSupport::TestCase`
since Rails 2.3.
See https://edgeguides.rubyonrails.org/2_3_release_notes.html#other-railties-changes

Since we already bumped the minimum version of MySQL to 5.5.8 at #33853.

[ci skip]

Since the `current_role` is public API.

Since the `preventing_writes?` is public API.

If exist `:nodoc:` before method define, it affects all subsequent
method definitions.

That ability was introduced at #11898 as `Relation#update` without
giving ids, so the ability on the class level is not documented and not
tested.

c83e30da which fixes #33470 has lost two undocumented abilities.

One has fixed at 5c656889, but I missed the ability on the class level.

Removing any feature should not be suddenly happened in a stable version
even if that is not documented.

I've restored the ability and added test case to avoid any regression in
the future.

Fixes #34743.

This reverts 27c6c071 since `arel_attr.to_s` is not right way to avoid
the type error.

That to_s returns `"#<struct Arel::Attributes::Attribute ...>"`, there
is no reason to match the regex to the inspect form.

And also, the regex path is not covered by our test cases. I've tweaked
the regex for redundant part and added assertions for the regex path.

[ci skip]

Add examples describing error handling in ActiveSupport::Notification…

Rename header of the gzip paragraph

Mention tiff support in ActiveStorage guides

[ci skip]

Add CHANGELOG.md to actionmailbox gem specification

Bump license years for 2019

Add streams assert methods to ActionCable channel test case