I reverted the wrong commit. Damn it.

This reverts commit f66a977f.

We had a discussion on the Core team and we don't want to expose this information
as a JSON endpoint and not by default.

It doesn't make sense to expose this JSON locally and this controller is only
accessible in dev, so the proposed access from a production app seems off.

This reverts commit 8eaffe7e, reversing
changes made to b6e4305c.

Generally followed the pattern for https://github.com/rails/rails/pull/32034

* Removes needless CI configs for 2.4
* Targets 2.5 in rubocop
* Updates existing CHANGELOG entries for fewer merge conflicts
* Removes Hash#slice extension as that's inlined on Ruby 2.5.
* Removes the need for send on define_method in MethodCallAssertions.

Reword first sentence of dep management and CVE section of
security guide. Also, reword and move gemspec notes above deps.

[ci skip]

[ci skip]

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

References #32028

Rails 6 will only support Ruby >= 2.3.

Without this change, bundler will resolve a gemfile which is pointing at
Rails master and not arel master. The error message that someone will
get from doing this will be incredibly unhelpful, as it'll be the result
of ActiveRecord being in a half-loaded that halted at `require
"arel/collectors/composite"`, which was subsequently rescued in
`"rails/all"`

This reverts commit 3420a145, reversing
changes made to afb66a5a.

".. with __dir__ we can restore order in the Universe." - by @fxn

Related to 5b8738c2

The current code base is not uniform. After some discussion,
we have chosen to go with double quotes by default.

References https://github.com/rails/homepage/issues/46.

and fix the grammar in the ruby_version_check.rb user message.

This is a follow-up to #19257

Stems from [this comment](https://github.com/rails/rails/pull/18203#issuecomment-68138096) by @robin850
and by the blog post http://weblog.rubyonrails.org/2014/12/19/Rails-4-2-final

[This article](http://weblog.rubyonrails.org/2014/8/20/Rails-4-2-beta1/#maintenance-consequences-and-rails-5-0) states that:

> Rails 5.0 is in most likelihood going to target Ruby 2.2.

Before the exact minimum version is fully decided, @arthurnn [suggests](https://github.com/rails/rails/pull/17830#issuecomment-64940383)
that **at least** version 2.1.0 **must** be required by the `gemspec` files.

This is a follow-up to 3053fee9.

> The `master` branch should use the `master` branch.

They were deprecated in 4.0, planned to remove in 4.1

associations

This reverts commit c24528fb.