Track Symfony 7 upgrade #351

This tracks the Symfony 7 upgrade for OpenConext-profile.

Context:

• PR chore: upgrade to PHP 8.5 #349 upgrades the project to PHP 8.5 and refreshes Docker, Composer, and npm dependencies.
• The Symfony runtime in that PR remains on the 6.4 line (symfony/* constraints and extra.symfony.require are still 6.4.*).

Follow-up work for this issue:

• Plan the Symfony 6.4 -> 7.x upgrade
• Resolve remaining Symfony deprecations and compatibility gaps
• Update docs and CI as needed for the Symfony major bump