Bump Symfony 6 support to ^6.3

[cavasinf]

As Symfony/* 6.2 is not maintained anymore.

[what-the-diff[bot]]

PR Summary

• Updated the Workflow to Use Symfony 6.3 The configuration for our continuous integration workflow, specifically in the .github/workflows/ci.yaml file, has been adjusted to now use the latest version of the Symfony framework, which is version 6.3. This change ensures that our workflow is built to operate with the most recent, and therefore likely the most robust, edition of Symfony.

• Upgraded Symfony Dependencies in composer.json Similary, the dependencies in our composer.json file have been updated to require Symfony version 6.3. Simply, this means that the various bits of software our project relies on will now all be compatible with, and utilise the advantages of, the newest version of Symfony.