Add Symfony 6 support

[jordisala1991]

Subject

I am targeting this branch, because this is BC.

Part of: sonata-project/dev-kit#1675
Dev-kit PR: sonata-project/dev-kit#1816

Changelog

### Added
- Added support for Symfony 6.
### Removed
- Removed support for Symfony 5.2.
- Removed support for PHP < 7.3.

[jordisala1991]

Blocked by: doctrine/phpcr-odm

[VincentLanglet]

doctrine/phpcr-odm

They only use symfony/console and symfony/yaml, maybe the PR could be easy.
If not, we might consider adding the support to at least not block orm or mongo.

[SonataCI]

Could you please rebase your PR and fix merge conflicts?

[VincentLanglet]

Waiting for doctrine/phpcr-odm#822

[VincentLanglet]

Since this package is deprecated, this might be not worthy to add SF6 @jordisala1991

[dbu]

phpcr-odm symfony 6 support is tagged as https://github.com/doctrine/phpcr-odm/releases/tag/1.6.0

[jordisala1991]

Since this package is deprecated, this might be not worthy to add SF6 @jordisala1991

Since it is that easy to add support I think it is better to add it to not have blockers for PageBundle.