fix(core): Get rid of redundant logback dependency

[flipp5b]

For now, core depends on logback. It's pretty uncommon for a library to depend on a concrete slf4j implementation (usually, it's up to the end application to decide which implementation to use).

This PR removes redundant logback dependency from the core and adds it to examples.

P.S. I'm new to Mill, so that may be not an idiomatic way to solve the issue 😅

[ex0ns]

Thanks for the PR, I those changes should solve this specific issue.
By the way, you can test by running a few examples using the following command:

`mill -i __.examples.jvm[_].console`
new EchoBot("<Your telegram bot token>").run

And this works similarly for monix (replace jvm with monixjvm, ziojvm and catsjvm)

[flipp5b]

I've tested examples as follows:

mill examples.cats3jvm[_].run EchoBot <token>

I can't spot any issues, at least logging works.

[ex0ns]

@flipp5b this is available in release 5.8.1 that should soon be available