To avoid word wrap in the rendered guide.

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.

Context https://github.com/rails/rails/pull/33605#discussion_r210354278

Related to #32937, #33605

This commit adds a skeleton of "Ruby on Rails 6.0 Release Notes".
It isn't a good time to add changelogs' entries to this guide since we can
redo/revert some things till the final release 6.0.
It would be better to do it close to the release.
But we already can add mentions about major features
that have been added to 6.0. I added mention about "Parallel Testing".

Update Rails 3.2 to 4.0 upgrade guide to include a mention and tip for handling the addition of configurable default HTTP headers. [ci skip]

http links will be redirected to the https version, but still better to
just directly link to the https version.

This commit substitutes references to rails/rake task for rails command

As discussed in #33203 rails command already looks for, and runs,
bin/rails if it is present.

We were mixing recommendations within guides and USAGE guidelines,
in some files we recommended using rails, in others bin/rails and
in some cases we even had both options mixed together.

Today there are two common ways for Rails developers to force their
applications to communicate over HTTPS:

* `config.force_ssl` is a setting in environment configurations that
  enables the `ActionDispatch::SSL` middleware. With this middleware
  enabled, all HTTP communication to your application will be redirected
  to HTTPS. The middleware also takes care of other best practices by
  setting HSTS headers, upgrading all cookies to secure only, etc.
* The `force_ssl` controller method redirects HTTP requests to certain
  controllers to HTTPS.

As a consultant, I've seen many applications with misconfigured HTTPS
setups due to developers adding `force_ssl` to `ApplicationController`
and not enabling `config.force_ssl`. With this configuration, many
application requests can be served over HTTP such as assets, requests
that hit mounted engines, etc. In addition, because cookies are not
upgraded to secure only in this configuration and HSTS headers are not
set, it's possible for cookies that are meant to be secure to be sent
over HTTP.

The confusion between these two methods of forcing HTTPS is compounded
by the fact that they share an identical name. This makes finding
documentation on the "right" method confusing.

HTTPS throughout is quickly becomming table stakes for all web sites.
Sites are expected to operate over HTTPS for all communication,
sensitive or otherwise. Let's encourage use of the broader-reaching
`ActionDispatch::SSL` middleware and elminate this source of user
confusion. If, for some reason, applications need to expose certain
endpoints over HTTP they can do so by properly configuring
`config.ssl_options`.

Add section "Expiry in signed or encrypted cookie is now embedded in the cookies values"
to `master` since it should always be in the guides, not only for version 5.2.

Add info about `config.action_dispatch.use_authenticated_cookie_encryption`
to the "Configuring Rails Applications" guide.

It was committed straight  to `5-2-stable` since we don't need this
functionality in 6.0. Related to b25fcbc0.

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

References #32028

The initial commit (efaa6e4f) that changed this
behavior was intended to be a minor change, but ended up becoming a
large-ish breaking change within Active Record.

This is because instead of only JSON encoding `Hash`es or `Array`s in `#serialize`,
we now encode all values passed in. This is an issue if you're passing in a `String`,
that has already been transformed from a `Hash` to a `String`, since your data
is now being double encoded.

Unfortunately, the change was included in one of the v5.0.0 beta
releases, and it is too late to revert without huge ripple effects.
Thus, all we can do is update the documentation (via this commit), and
add some test coverage (coming soon in a PR) for the new behavior.

Please note that in the documentation I talk about deserialization, not
about serialization, where the actual change occurred. This is because
you won't notice any changes in serialized data until you try and
deserialize it. Also to make the change itself (confusing until you
read through everything multiple times) easier to understand.

Related #27788, #25594, #26101, #24234, #28292, #28285, #28285, and
probably others.

[ci skip]

`it` and `has` are for singular, not plural

[ci skip]

Updates Rails upgrade guide for 4.2 to 5.0 regarding the removal of `ActionView::Helpers::RecordTagHelper`

Update upgrading guide w.r.t. Parameters to use other example method than slice as this has actually been implemented by Parameters