Plan for the next major release v4.0.0

[GromNaN]

A new major release is necessary in order to be compatible with Symfony 8 (#510 (comment)).

A major release is the only opportunity we have to make breaking changes.
Here is the list of things to do:

• Add support for Symfony 8: Add support for Symfony 8 #510 (comment)
• Drop support for Symfony < 7
• Drop support for Monolog < 3 Drop support for Monolog 1 #494
• Remove SwiftMailer\MessageFactory, as Swiftmailer is not supported since Monolog 3
• Remove DebugHandlerPass
• Remove excluded_404s option
• Remove elasticsearch type
• Add return types to every classes
• Reorganization of files to follow the directory structure
• Convert config from XML to PHP #509
• Run tests using phpunit

The branch 3.x of the MonologBundle will be maintained until the EOL of Symfony 6.4.