This is an alternative of #31877 to fix #31876 caused by #28808.

This issue was caused by a combination of several loose implementation.

* finding automatic inverse association of polymorphic without context (caused by #28808)
* returning `klass` even if `polymorphic?` (exists before #28808)
* loose verification by `valid_inverse_reflection?` (exists before #28808)

This makes `klass` raise if `polymorphic?` not to be misused.
This issue will not happen unless polymorphic `klass` is misused.

Fixes #31876.
Closes #31877.

3acc5d6e was changed the order of scope evaluation from through scope to
the association's own scope to be prioritized over the through scope.
But the sorting order will be prioritized that is evaluated first. It is
unintentional effect, association scope's sorting order should be
prioritized as well.

Fixes #32008.

Fix active_job_basics.md callbacks example [ci skip]

This reverts commit cf4f05a7.

Since Rails 6 requires Ruby 2.4.1+.

So `2.4` would match `2.4.1` (due to an rvm alias) rather than matching
the latest `2.4.3` release.

This faithfully preserves grapheme clusters (characters composed of other
characters and combining marks) and other multibyte characters.

```ruby
"foo".freeze.strip_heredoc.frozen?  # => true
```

Fixes the case where frozen string literals would inadvertently become
unfrozen:

```ruby

foo = <<-MSG.strip_heredoc
  la la la
MSG

foo.frozen?  # => false !??
```

Prepare bug report templates for Rails 6.0 development

Add missing `require "active_support"` in `guides/bug_report_templates/generic_gem.rb`

Fixes typos

Skipping over 2.4.0 to sidestep the `"symbol_from_string".to_sym.dup` bug.

References #32028

Object#blank? used to be used in this file, but it's not anymore.

This avoids a monkey-patch, for those who want to use just this isolated
feature of ActiveSupport.

* Global ignores at toplevel .gitignore
* Component-specific ignores in each toplevel directory
* Remove `actionview/test/tmp/.keep` for JRuby

```
rm actionview/test/tmp/ -fr
cd actionview/
bundle exec jruby -Itest test/template/digestor_test.rb
```

Related to #11743, #30392.

Closes #29978.

Closes #31998

Some attr_readers should be `protected` instead of `private`

See https://travis-ci.org/rails/rails/builds/342800276

See https://github.com/ruby/ruby/blob/ruby_2_3/NEWS

Multipart file uploads are very rare in API only apps so don't include Rack::TemfileReaper in default middleware stack for API only apps

Fix custome serializer setting

References 89bcca59

Use SuppressedSummaryReporter and Rails::TestUnitReporter only if needed

Fix array routing constraints

Make sure that when transforming the keys of a HashWithIndifferentAccess
we can still access with indifferent access in Ruby 2.5.

Closes #32007.

Remove needless requiring 'active_support/core_ext/string/strip'

References 89bcca59

It seems to debug print.

Some places we can't remove because Ruby still don't have a method
equivalent to strip_heredoc to be called in an already existent string.

Rails 6 will only support Ruby >= 2.3.

PERF: reduce retained objects in Journey

ActiveStorage file cleanup in Integration Tests

Parallel testing