Add doctrine/orm 3 support

[franmomu]

Subject

This PR tries to add support for doctrine/orm 3

I am targeting this branch, because these changes are BC.

Changelog

### Added
- Support for `doctrine/orm` 3

The CI should be modified to allow installing doctrine/orm 3

[franmomu]

We need doctrine/data-fixtures#452

[greg0ire]

The next blocker seems to be doctrine-extensions/DoctrineExtensions#2708

[franmomu]

The next blocker seems to be doctrine-extensions/DoctrineExtensions#2708

Yep, that would take a bit longer I think, I'll try to help with that

[VincentLanglet]

Hi @franmomu, what about following the same strategy than https://github.com/sonata-project/SonataDoctrineORMAdminBundle/blob/4.x/.github/workflows/test_orm_3.yaml ?

gedmo/doctrine-extensions is an optional dev dependency, it should be a blocker for the ORM 3 support.

[amacrobert-meq]

Is there anything I can do to help this PR along? This bundle is holding up an ORM upgrade on our end, so I'm happy to help if I can.

[VincentLanglet]

Is there anything I can do to help this PR along? This bundle is holding up an ORM upgrade on our end, so I'm happy to help if I can.

Basically, we need green tests.

If you want you can recreate the same PR and add a github actions similar to https://github.com/sonata-project/SonataDoctrineORMAdminBundle/blob/4.x/.github/workflows/test_orm_3.yaml

Instead of removing AuditBundle you remove the doctrineExtensions.