Drop support for Symfony < 4.3

[wbloszyk]

Subject

Drop support for Symfony < 4.3.

I am targeting this branch, because this change respect BC.

Changelog

### Removed
- Remove support for Symfony <4.3 and php <7.2
### Fixed
- Deprecations for event dispatching

[jordisala1991]

Can you fill the PR template?

[wbloszyk]

Should i drop symfony 4.3 too?

[jordisala1991]

Is it worth it?

[wbloszyk]

Symfony 4.3 is not maintained. I think we can keep support for 4.3 and drop in in master.

[jordisala1991]

That would be good I think. Make all BC changes on 3.x. Upgrade to 4.4 and bc break on master

[core23]

IMHO you can bump this to 4.4, as the 4.3 support will end in a few weeks:
https://symfony.com/releases/4.3

[jordisala1991]

Thank you @wbloszyk

[wbloszyk]

@core23 I know but diffrence is not do much. Also all sonata 3 bundles support it. It will be good idea to keep minimum require symfony 4.3 in whole sonata 3.

[core23]

What I wanted to say is that our release cycle is very slow for this bundle. IMHO you can already create a new PR to drop 4.3.

[wbloszyk]

I will focus on sandbox and PageBundle now. I needed in to my 3 commerce project. I have many improve in my roadmap for this bundle too. I will update some issue on next week like sonata 4, blocks upgrade, sandbox 3 and 4 roadmap, ecommerce.

[greg0ire]

Should this be patch or minor?

[wbloszyk]

Next release should be minor and should include #678. This PR is RTM. Will be awesome release it with drop CoreBundle (both drop twig/extensions).