Move types to the top level `ActiveRecord` namespace

`ActiveRecord::ConnectionAdapters::Type::Value` =>
`ActiveRecord::Type::Value`
pg, keep `hstore` and `json` attributes as `Hash` in @attributes.

The solution presented in this patch is not efficient. We should replace it
in the near future. The following needs to be worked out:
  * Is `@attributes` storing the Ruby or SQL representation?
  * `cacheable_column?` is broken but `hstore` and `json` rely on that behavior

Refs #15369.

/cc @sgrif @rafaelfranca
Initial doc for TransactionIsolationError [ci skip]
Add with_default_charset helper.
Merge pull request #15381 from zuhao/refactor_actionpack_content_type_test

Add with_default_charset helper.
Move I18n.locale setting into setup and teardown.
Merge pull request #15383 from zuhao/refactor_actionpack_localized_templates_test

Move I18n.locale setting into setup and teardown.
Add and remove renderer inside the test to prevent leak.
Avoid hardcoded value in teardown.
Merge pull request #15385 from zuhao/refactor_actionpack_render_other_test

Add and remove renderer inside the test to prevent leak.
Remove unused `Column#coder`

It appears this property was added, but never actually used. It would be
broken if it were, as it only type casts one way.
Unregister Mime::Type in teardown.
Merge pull request #15388 from zuhao/refactor_actionpack_send_file_test

Unregister Mime::Type in teardown.
Merge pull request #15387 from sgrif/sg-remove-column-coder

Remove unused `Column#coder`
Merge pull request #15386 from zuhao/refactor_actionpack_request_forgery_protection_test

Avoid hardcoded `request_forgery_protection_token` value in teardown.
Clear inflections after test.
Revert "Merge pull request #12778 from etehtsea/rake-default-fix"

This reverts commit ec0a2b57f67c9153cb5f7bbe2a3f66d13fe64bdd, reversing
changes made to a8bd7b16260c217290ef8fe3a166fcceac442c62.

This reverts commit 555ec36522011862c03b483c53be32410594a51e

This reverts commit 555ec36522011862c03b483c53be32410594a51e

By default, Rails uses the `default` Rake task to run all tests.
This commit changed the environment of the default task to `development`.
This clears the development database and has other negative consequences.
Merge pull request #15380 from Gaurav2728/gaurav-transaction_isolation_error

Initial doc for TransactionIsolationError [ci skip]
Formatting of content from b1db615 [ci skip]
Upgrading Ruby on Rails guide is no longer WIP [ci skip]