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 Require PHP 8.2, Symfony 7.3 and Monolog 3 #515
• Drop support for Monolog < 3 Drop support for Monolog < 3.5 #494
• Remove SwiftMailer\MessageFactory, as Swiftmailer is not supported since Monolog 3
• Remove DebugHandlerPass
• Remove excluded_404s option Remove deprecated code #526
• Remove elasticsearch type Remove deprecated code #526
• Add return types to every classes
• Reorganization of files to follow the directory structure Reorganize files to match the "Reusable Bundles" structure #516
• Convert config from XML to PHP #509
• Run tests using phpunit Run the tests with PHPUnit 11 #520
• Remove raven type, already deprecated and replaced by sentry.

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

[HypeMC]

The raven type has been deprecated for a while and replaced with sentry, I think it'd be a good idea to remove it as well.

[HypeMC]

It looks like two libraries for Gelf are currently supported, one of which is abandoned: https://packagist.org/packages/mlehner/gelf-php. Support for that library should be removed as well.

[GromNaN]

All done, thank you @HypeMC and @derrabus.