Revert method documentation moved from migrations.md  to active_record_migrations.md

Reversible method documentation moved from migrations.md  to active_record_migrations.md

[Joshua Cody & Yves Senn]

Closes #16757.

Prior to this patch schema loading rake tasks had the potential to leak a
connection to a different database. This had side-effects when rake tasks
operating on the current connection (like `db:seed`) were chained.

Fix for double ActiveJob::DeserializationErorr

reset_column_infomation shouldn't be part of the assertion.

use correct operator in query based on JSON document [ci skip]

Move column option handling to new_column_definition

TableDefinition#column is not called from `add_column`.
Use TableDefinition#new_column_definition for column option handling.

Fix failing tests in ActiveJob Adapter

Remove 'if exists' from drop table statement then use `table_exists?`

Since 'drop table if exists' statement does not always work with
some databases such as Oracle.

Is this a thing we want?

Note current duplication between the `assert_select` section here, and
the corresponding release note entry... does this detail *move* from
there to here?

[ci skip]

Refactor add_constraints complex loop conditional

Shorten ActionView::Base doc summary line. [ci skip]

Clarify Rails uses erubis not stdlin ERB. [ci skip]

Remove '*' which doesn't belong [ci skip]

I found an * does not work in Markdown notation. It should be removed.

Add and Remove string/strip require

Method .strip_heredoc is defined in
active_support/core_ext/string/strip.rb so we need to require it.

[fixes #16677]

[ActiveJob] TestCase

We're seeing too many failures to believe otherwise.

This reverts commits bc116a55,
cbde413d,
bf0a6793, and
2440933f.

ActiveJob: Implemented enqueue_at for QueueClassic

[ci skip] Fix a typo in the doc of MessageDelivery

Do not swallow exception on NameError within constant.

Now we keep only the common code and move the specific code to the gems