Standardize on `Rails.application` [ci skip]

This seems to be the style settled on in most of the templates.
[ci skip] modify the description of production log level default
Merge pull request #16988 from y-yagi/chg_default_log_level

[ci skip] modify the description of production log level default
Fix typo [ci skip]
Merge pull request #16994 from yuki24/fix-typo-in-ar-attribute-method-test

[ci skip] Fix typo in activerecord/test/cases/attribute_methods_test.rb:737
Merge pull request #16979 from square/standardize-call-style

Standardize on `Rails.application` [ci skip]
Document that the default for `rails server -b` has changed

Fixes #16578
Merge pull request #16984 from chancancode/fix_16578

Document that the default for `rails server -b` has changed
Added RDoc for each Active Job adapter