Both `mysql2/json_test.rb` and `postgresql/json_test.rb` have same test
cases.

[ci skip]

Simplify handling of prerequisites in Getting Started guide [ci skip]

Merge pull request #29072 from dixpac/dixpac/add_documentation_for_validate_and_callback_order_of_execution

Improving docs for callbacks execution order [ci skip]

Telling people about prerequisites, and then telling them a page later
how to check and install those prerequisites, is confusing. This commit
removes the list and just handles the software installation in one
place.

Fixes #28565

When define callbacks latest definition on the same callback/method
overwrites previous ones.

Fix implicit calculations with scalars and durations

Remove obsolete Guides source files [ci skip]

* Nested Model Forms guide is out of date, not linked from index,
  and material is covered in the Form Helpers guide.
* Profiling guide was committed as an outline years ago
  and never actually written.

Fix broken external link in security guide.

see: https://github.com/mperham/sidekiq/blob/master/5.0-Upgrade.md

Fix typo in guides [ci skip]

Make `VALID_DIRECTIONS` to `Set`

Previously calculations where the scalar is first would be converted
to a duration of seconds but this causes issues with dates being
converted to times, e.g:

    Time.zone = "Beijing"           # => Asia/Shanghai
    date = Date.civil(2017, 5, 20)  # => Mon, 20 May 2017
    2 * 1.day                       # => 172800 seconds
    date + 2 * 1.day                # => Mon, 22 May 2017 00:00:00 CST +08:00

Now the `ActiveSupport::Duration::Scalar` calculation methods will try
to maintain the part structure of the duration where possible, e.g:

    Time.zone = "Beijing"           # => Asia/Shanghai
    date = Date.civil(2017, 5, 20)  # => Mon, 20 May 2017
    2 * 1.day                       # => 2 days
    date + 2 * 1.day                # => Mon, 22 May 2017

Fixes #29160, #28970.

Update 'Rails on Rack' guide [ci skip]

* Adjust middlewares list to match current defaults
* application.routes runs on application object, not Rails
* Add explanation of Sprockets::Rails::QuietAssets

Add :json type to auto_discovery_link_tag

This allows auto_discovery_link_tag to support the JSON Feed standard.
See https://jsonfeed.org/version/1 for more information.

Fix `warning: assigned but unused variable - timestamp`

```ruby
require "benchmark/ips"
require "set"

array = [:asc, :desc, :ASC, :DESC, "asc", "desc", "ASC", "DESC"]
set   = array.to_set
item  = "DESC"

Benchmark.ips do |x|
  x.report "array" do
    array.include?(item)
  end
  x.report "set" do
    set.include?(item)
  end
end
```

```
% ruby array_vs_set.rb
Warming up --------------------------------------
               array   188.441k i/100ms
                 set   229.531k i/100ms
Calculating -------------------------------------
               array      3.508M (± 9.0%) i/s -     17.525M in   5.043058s
                 set      5.134M (± 7.6%) i/s -     25.707M in   5.038921s
```

Add documentation to accessors in AbstractController::Base

Suppress `warning: assigned but unused variable - stdout`

This retains the existing behavior of
ActiveSupport::Cache.expand_cache_key (as used by etaging) where the
cache key includes the version.

see: https://bugs.ruby-lang.org/issues/6696

* Fix indentation.
* Add backticks.

[ci skip] explain async queue and rake tasks