Add compatibility for Symfony 6

[alexander-schranz]

Q	A
Bug fix?	no
New feature?	yes
BC breaks?	no
Deprecations?	no
Fixed tickets	fixes #issuenum
Related issues/PRs	#issuenum
License	MIT

What's in this PR?

Add compatibility for Symfony 6.

Why?

Compatiblity to Symfony 6 and get ride of deprecated swiftmailer.

[rogamoore]

Would it be possible to release these changes? I noticed that currently our projects are unable to update to Sulu 2.5 because it fails with

!!    Compile Error: Declaration of Sulu\Bundle\FormBundle\Controller\FormWebsite
!!    Controller::getSubscribedServices() must be compatible with Sulu\Bundle\Web
!!    siteBundle\Controller\WebsiteController::getSubscribedServices(): array

From the changes here I guess that should be fixed already. :)

[alexander-schranz]

@rogamoore we want to first test the demo with all upgraded bundles before doing a stable release of the bundles. If you already want to use you can install this via using ^2.5@dev in your composer.json for the sulu/form-bundle.