Automatically guess the inverse associations for STI

ActiveRecord associations automatically guess the inverse associations.
But this feature does not work correctly on assoctions for STI.
For example, before this commit

```
class Post < ActiveRecord::Base
  belongs_to :author
end

class SpecialPost < Post; end

class Author < ActiveRecord::Base
  has_many :posts
  has_many :special_posts
end
```

`author.posts.first.author` works correctly, but
`author.special_posts.first.author` does not work correctly.

missing backquotes [ci skip]

[actionpack] Update links to use https link instead of http [ci skip]

[activerecord] Update links to use https instead of http [ci skip]

Remove frozen_string_literal magic comment from templates

[activesupport] Update links to use https instead of http [ci skip]

Update MIT licenses link [ci skip]

Closes #30134.

Update debugging rails applications guide [ci skip]

[ci skip] form_with in the getting started guide.

Add back a bit about a resource oriented style of routing.

Fix broken link to mysql docs in database.yml [ci skip]

Fix link in Contributing Ruby on Rails guide [ci skip]

* Add Contributing to the Rails Code link.
* Wiki link for Spanish is redirect.
* The base branch of Polish repo is master. And Turkish is, too.
* Unified expression `rails-dev-box`
* The Clone url is wrong.
* The mailing list url is old.

Update Contributing to Ruby on Rails guide [ci skip]

Update generator guide [ci skip]

`SET time zone 'value'` is an alias for `SET timezone TO 'value'`.

https://www.postgresql.org/docs/current/static/sql-set.html

So if `variables["timezone"]` is specified, it is enough to
`SET timezone` once.

Update Rails on Rack guide [ci skip]

Make `restart` and `dev:cache` tasks work when customizing pid file path

Update Autoloading and Reloading Constants guide [ci skip]

Originally, it hard-coded pid file path. It can not be removed when customizing
pid file path.
But rake task can not get pid file path. Therefore, do not remove file in rake
task, makes it possible to judge whether it is restart from the argument of the
command and removes the file in server command.

Fixes #29306