* Use `gem 'redis', '~> 4.0'` for new app Gemfiles
* Loosen Action Cable redis-rb dep to `>= 3.3, < 5`
* Bump redis-namespace for looser Redis version dep
* Avoid using the underlying `redis.client` directly
* Use `Redis.new` instead of `Redis.connect`

Break long variant filenames into path chunks

Exception message for SystemTestCase#get etc..

This is preparation to respect parent relation's alias tracking for
fixing #30681.

Fix radio_button_tag comment

Fix default of `String#to_time` [ci skip]

The default of `String#to_time` is `:local` since b79adc43.

Colons, periods, etc. can also be included in id.
The sanitize_to_id method does not remove them.

[ci skip] Fix typo

Update Instrumentation guide for Active Storage [ci skip]

Remove needless setup from `InfoTest`

The `abstract_unit` loads `rails/all`. Therefore, `Rails` is definitely defined.
Also, since `Info` is autoloaded, do not need to explicitly load the file.
https://github.com/rails/rails/blob/3ede539357acc91e377611cea9dd5f30678e7b2a/railties/lib/rails.rb#L29

Add v5.1 to earlier releases list in guide [ci skip]

Missing backquotes in the guides

If a variant has a large set of options associated with it, the generated
filename will be too long, causing Errno::ENAMETOOLONG to be raised. This
change replaces those potentially long filenames with a much more compact
SHA256 hash. Fixes #30662.

link tags shouldn't depend on the request

Use __callee__ to pass alias instead of original method name

ActionCable: use find method when unsubscribing

Before

```
> Article.left_joins
ArgumentError: The method .left_outer_joins() must contain arguments.
```

After

```
> Article.left_joins
ArgumentError: The method .left_joins() must contain arguments.
```

Implement H2 Early Hints for Rails

When puma/puma#1403 is merged Puma will support the Early Hints status
code for sending assets before a request has finished.

While the Early Hints spec is still in draft, this PR prepares Rails to
allowing this status code.

If the proxy server supports Early Hints, it will send H2 pushes to the
client.

This PR adds a method for setting Early Hints Link headers via Rails,
and also automatically sends Early Hints if supported from the
`stylesheet_link_tag` and the `javascript_include_tag`.

Once puma supports Early Hints the `--early-hints` argument can be
passed to the server to enable this or set in the puma config with
`early_hints(true)`. Note that for Early Hints to work
in the browser the requirements are 1) a proxy that can handle H2,
and 2) HTTPS.

To start the server with Early Hints enabled pass `--early-hints` to
`rails s`.

This has been verified to work with h2o, Puma, and Rails with Chrome.

The commit adds a new option to the rails server to enable early hints
for Puma.

Early Hints spec:
https://tools.ietf.org/html/draft-ietf-httpbis-early-hints-04

[Eileen M. Uchitelle, Aaron Patterson]

Encourage html-safe API in layouts/rendering guide

Fix formatting of `Time.use_zone` [ci skip]

* Add missing credit
* Add backticks
* Fix indentation
* Remove trailing spaces

And some minor tweaks.

Fix third-party system libraries list in ActiveStorage::Preview [ci skip]

Add test case for `font_url`

Since test of `font_url` was not in Action View's test suite, so it added.