Update README extension in Getting Started guide

Generated Rails app READMEs are Markdown as of 9739f07d763e29b1c5d71cabf1ca8cfa4421e653

Add missing @claudiob credit to change log [skip ci]

Remove legacy mysql adapter

Follow up to #22642.

Upgrade Guide: Add missing period, ✂ whitespace [skip ci]

mysql2 adapter instead of mysql [ci skip]

Suppress warning ambiguous first argument

Suppress warning (warning: ambiguous first argument; put
parentheses or a space even after `/' operator)

It is in the code to provides backward compatibility for people that
have this class serialized as YAML in some storage.

Closes #22681

Remove unused `test/schema/mysql_specific_schema.rb`

Update README.md

Follow up to #22642.

Also, added a test case to make sure that the default deprecation horizon is
always bigger than the current Rails version.

Remove .DS_Store

Fix cache fetch miss notification order

Add No Doc

Remove blank line generated in config.ru

[ActionCable] declare asset_logged private

Explain what initializer/config_serializer does

Follow the lines of the other `.tt` files ([example](https://github.com/rails/rails/blob/1611ab4db173a7596a7a94c58dabf1483f070304/railties/lib/rails/generators/rails/app/templates/config/environments/development.rb.tt#L26)) that have the space after the
condition to avoid too many white lines in the resulted file.

In reference to rails/rail#22403 add a :nodoc:

In line with the other initializers, add a short comment that explains
what happens when you change the value of the initializer.

[ci skip]

initializers/cookies_serializer is not new to 5.0

[ci skip]

The initializer has existed since 4.1, for instance see:
https://github.com/rails/rails/blob/v4.1.0/railties/lib/rails/generators/rails/app/templates/config/initializers/cookies_serializer.rb

Merge pull request #19423 from yuroyoro/fix_performance_regression_of_preloading_has_many_through_relation

Fix #12537 performance regression when preloading has_many_through association

fix typo in config value [ci skip]

Fix typo in CHANGELOG