Drop support for Symfony 4

[derrabus]

Active support for Symfony 4 ends this month. This PR drops compat code we have maintained for Symfony 4.

[dmaicher]

I think we should then also remove all of those?

https://github.com/doctrine/DoctrineBundle/blob/2.8.x/Resources/config/dbal.xml#L26-L37

This would then also fix #1585

[derrabus]

I think we should then also remove all of those?

There are tests in place that assert that the doctrine.dbal.logger service exists even if we use the logger middleware. But of course, that service serves no purpose anymore. It's just there. If removing it is not considered a BC break, I can remove it and adjust the tests.

[ostrolucky]

People will be upset about removing this service for sure, but better to throw it in their face that they need to change something, rather than their code not doing anything. So I agree with removal.

[stof]

Symfony supports marking a service as deprecated. That's what we should do for the logger service when we use the middleware

[dmaicher]

Symfony supports marking a service as deprecated. That's what we should do for the logger service when we use the middleware

But with this PR we will always use the middlewares. So the legacy logging services are not doing anything anymore. You would prefer to still keep them and deprecate them?

EDIT: I personally would be fine with removing them. I see those as internal details and not as an extension point that one should rely on.