documentation fix

[ci skip] Don’t explicitly mention EventMachine

Since #23152 eliminated the EventMachine dependency, we don’t need to
explicitly mention EventMachine.

Nevertheless, I'm not 100% sure about saying "the websocket-driver loop"
driver… any suggestions, @matthewd or @pixeltrix ? 😅

[ci skip]

Update `Gemfile.lock`

* 5-0-beta-sec:
  bumping version
  fix version update task to deal with .beta1.1
  Eliminate instance level writers for class accessors
  allow :file to be outside rails root, but anything else must be inside the rails view directory
  Don't short-circuit reject_if proc
  stop caching mime types globally
  use secure string comparisons for basic auth username / password

Fix nodoc to internal class error document some of them

[ci skip]
Fixes #20808

[Vipul A M & Julio Lopez]

[close #23009] Limit key length

Pass 2 over testing guide

When verifying size of relation, also verify count is ok.

Change permission to readonly [ci skip]

Add missing source_type if provided on hmt which belongs to an sti re…

Fix AR::Relation#cache_key to remove select scope added by user

This reverts commit 62aa850f, reversing
changes made to 8c1f248c.

There is no reason to disallow mail 2.5 so we don't need to bump the
version constraint since people are still able to use mail 2.6 and get
all the memory saving that was pointed in the pull request description.

ActionCable: Allow adding custom paths for channels

- We don't need the select scope added by user as we only want to max
  timestamp and size of the collection. So we already know which columns
  to select.
- Additionally having user defined columns in select scope blows the cache_key
  method with PostGreSQL because it needs all `selected` columns in the group_by
  clause or aggregate function.
- Fixes #23038.

Fixes #23209

Bump mail gem constraint from [~> 2.5, >= 2.5.4] to ~> 2.6

Fix ActiveRecord::Relation#cache_key for relations with no results

Be consistent in testing outputs from railties test and use /bin/rails

Eliminate the EventMachine dependency

Update the exception of format constraint in routes

This fixes ⚠ previous definition of download was here

Be consistent in testing outputs from railties test and use /bin/rails everywhere(the default behaviour now) instead of mix of /bin/rake /bin/rails everywhere
[Ryo Hashimoto & Vipul A M]

Mails downstream dependency (Mime-types) has been shown to decrease
memory usage significantly in its 3.0 release. This memory decrease
will be a big win for users upgrading to Rails 5.

Lets nudge users to upgrade Mail alongside Rails.

Pare back default `index` option for the migration generator

- Using `references` or `belongs_to` in migrations will always add index
  for the referenced column by default, without adding `index:true` option
  to generated migration file.
- Users can opt out of this by passing `index: false`.
- Legacy migrations won't be affected by this change. They will continue
  to run as they were before.
- Fixes #18146

We're no longer doing our work in the EM event loop, so errors are quite
unlikely, and if they do occur, they're not really our responsibility to
handle.

(as adapted to use concurrent-ruby / nio4r instead of eventmachine)

ActionCable Adapter tests