Make optional and deprecate the BackendChain constructor parameter

This was discovered while working on 📌 Remove drupal_static from BookManager Fixed .

Problem/Motivation

The BackendChain service constructor looks like:

public function __construct($bin) {
}

This is forcing a service definition to declare arguments, in *.services.yml, such as:

book.cache:
  class: Drupal\Core\Cache\BackendChain
  arguments: ['']
  ...   

or...

book.cache:
  class: Drupal\Core\Cache\BackendChain
  arguments: ['whatever arbitrary string...']
  ...   

The parameter is not used and not needed.

Proposed resolution

Make the parameter optional and deprecate it. Will be removed in 9.0.0.

Remaining tasks

None.

User interface changes

None.

API changes

The BackendChain constructor parameter is optional and deprecated.

Data model changes

None.

Release notes snippet

None.