require 'test_help' -> require 'rails/test_help'
[ci skip] Update plugin doc with the latest rails
Add missing require
Mention deprecation of assert_select and friends in the release notes [ci skip]
Sync Action View release notes [ci skip]
Sync Action Mailer release notes [ci skip]
Removed unnecessary word
[ci skip] config.active_record.errors_in_transactional_callbacks -> config.active_record.raise_in_transactional_callbacks
Merge pull request #16553 from tomkadwill/removed_word

[ci skip] Removed unnecessary word from Getting Started
http://gembundler.com/  =>  http://bundler.io/
4.2 Release Notes pass [skip ci]
Merge pull request #16552 from untidy-hair/update_plugin_guide

[ci skip] Update plugin doc with the latest rails
use same idiom to define class method as in every other adapter.
"warning: assigned but unused variable"
Merge pull request #16554 from y-yagi/patch-2

[ci skip] config.active_record.errors_in_transactional_callbacks -> config.active_record.raise_in_transactional_callbacks
Merge pull request #16556 from jonatack/patch-9

4.2 Release Notes pass [ci skip]