Update for Symfony 5 and 6

[GCalmels]

• Change to short array syntax (array() => [])
• Update dependencies
• Modify documentation

Warnings Not tested for ODM but a demo for ORM can be find here https://github.com/Kinoba/FOSMessageBundleDemo

[MehGokalp]

Hi @GCalmels thank you for sharing! Dear FOS developers, do you have any plan for merging this pr in future? Thank you in advance