Define connections in files

[brotkrueml]

Configuration should not be stored in the database, instead files should be used. This has advantages:

• Configuration and its history can be versioned via Git.
• Configuration is deployable.

TYPO3 v12 will probably provide a way to read configuration from files and hydrate value objects accordingly, see discussion:
https://decisions.typo3.org/t/configuration-overhaul-plan/728

Acceptance criteria:

• Connections are defined in yaml files. The following fields are available:
  - identifier (string, unique, required)
  - baseUrl (string, required)
  - username (string, required)
  - password (string, required)
  - enabled (bool, optional, defaults to true)
• An upgrade wizard to move the configuration from the database into yaml files is available.
• Configuration values, e.g. the password, can be read from environment variables. This ensures that no sensible data is stored in a file and so in Git.
• The backend module displays all available connections readonly. A connection check is possible.
• The identifier (formerly known as "handle") links another configuration (data, process) with a connection.
• The Connection model and the ConnectionRepository are removed.
• The Password evaluation class is removed.
• The documentation is adjusted.
• Add breaking entry to changelog.