[Maintenance] Remove outdated SF mention at GH and packagist repo description

[diimpp]

Mention of outdated versions makes feel as bundle itself is outdated, which is not true.

GH repo description

--- Symfony bundle integrating Flysystem into Symfony 4.2+ applications 
+++ Symfony bundle integrating Flysystem into Symfony applications 

Packagist/composer package description

--- Symfony bundle integrating Flysystem into Symfony 5.4+ applications
+++ Symfony bundle integrating Flysystem into Symfony applications

[maxhelias]

@tgalopin I updated the composer.json here : #169 but I don't have the permissions to do that on the repo, can you?

[tgalopin]

Done, good idea!