bump to Symfony 7

[tacman]

Or 6.4, but really Symfony 7. There were some assetmapper changes between 6.3 and 6.4, too. No reason to lock this repo at 6.3.

[Nispeon]

Indeed I do not think that there is any reason to keep this repo in 6.3, but upgrading to Sf 7 would not change much to this project and I feel it would be a big task with little purpose.

That said, if a PR was opened to take care of it, it'd be my pleasure to merge it 😄