Suggestion: Always create `deprecation` channel by default

[ThomasLandauer]

The current system of channels is confusing IMO:

1. There are "secret" channels like e.g. security that exist by default, but you don't see them in your config (config/packages/monolog.php).
2. Then there's a channel with a predefined name deprecation, but not enabled by default. Could be called "super-secret" cause if you missed the recipe at https://github.com/symfony/recipes/blob/main/symfony/monolog-bundle/3.7/config/packages/monolog.yaml you'll probably never find out that this channel exists (cause it's not shown at php bin/console debug:container monolog)
3. On top of all that, there are the channels that you define on your own in the config.

So I'm suggesting to enable deprecation by default (just like the built-ins like router etc.).