Fixes #33083

Given the change to using `bigint` as type for auto-generated primary keys (as per https://github.com/rails/rails/pull/26266), I propose updating the Active Record Basics guide to explicitly state the type being used so as to avoid confusion as to the type of integer used, especially as the type can vary depending on the database used.

I reverted the wrong commit. Damn it.

This reverts commit f66a977f.

We had a discussion on the Core team and we don't want to expose this information
as a JSON endpoint and not by default.

It doesn't make sense to expose this JSON locally and this controller is only
accessible in dev, so the proposed access from a production app seems off.

This reverts commit 8eaffe7e, reversing
changes made to b6e4305c.

- Move some actiontext/README.md content to Action Text Overview guide
- I added WIP label to that guide since we definitely want to complement it.
- Add Action Text to Major Features of Rails 6.0

Similar approach was used in #34812

See actiontext/lib/action_text/content.rb:132

[ci skip]

Match Action Mailbox, which sets a default queue for each of its two jobs.

Since #34356 logging `locale` value is more correct

Add section "Results of `load_defaults`" to the guide.

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

[ci skip]

[ci skip]

Now Action Mailbox is responsible for receiving emails.

I added WIP label to that guide since we definitely
want to complement it.

Since e3f832a7 `ActionMailer::Base.receive` is
deprecated.

Rename header of the gzip paragraph to more accurate and add short info aboaut what to do about serving gziped files [ci skip]

- Fix a few deprecation warnings
- Remove testing of `Hash#slice`
- Imporve test of `Hash#slice!`
- Remove mention about `Hash#slice` from the guide

Since Rails 6 requires Ruby 2.5.

https://github.com/ruby/ruby/blob/ruby_2_5/NEWS

Follow up #34754.

Generally followed the pattern for https://github.com/rails/rails/pull/32034

* Removes needless CI configs for 2.4
* Targets 2.5 in rubocop
* Updates existing CHANGELOG entries for fewer merge conflicts
* Removes Hash#slice extension as that's inlined on Ruby 2.5.
* Removes the need for send on define_method in MethodCallAssertions.

Double nesting of configuration is not supported (without using an
intermediate object), even though the docs suggest it is.

[ci-skip]

* Add option to set parallel test worker count to the physical core count of the machine

Also, use the physical core count of the machine as
the default number of workers, and  generate the `test_helper.rb` file
with `parallelize(workers: :number_of_processors)`

Closes #34734

* Ensure that we always test parallel testing

Since #34734 we decided to use the physical core count of the machine as
the default number of workers in the parallel testing, we need to
ensure that some tests use at least 2 workers because we could
run those tests on VM that has only 1 physical core.
It also fixes tests failures on the CI since Travis server we are using
has only one physical core.
See https://travis-ci.org/rails/rails/jobs/469281088#L2352

* Extend documentation of `ActiveSupport::Notifications.subscribe`

Add mention that a block with only one argument passed to the method
will yield an event object.

Related to #33451

* Emphasize that `SubscribeEventObjects` is a test class by adding suffix `Test`

[ci skip]

* No need to go through ruby

* Abort early if a generator command fails

* Reuse `rails_command` method

* Bump thor minimum dependency to 0.20.3

* Add some minimal docs

* Add a changelog entry

* Restore original logging

And make sure new applications in Rails 6.0 has this config enabled.

Also, improve test coverage and add a CHANGELOG entry.

[ci skip]

- Fix formatting
- Don't repeat "Active Record automatically handles creating and migrating a new
  database for each worker to use."
- Tell that AR loads the schema to a database for each process(Related to #33479)
- Clarify that `parallelize_teardown` is executed for each process

[ci skip]