Update to Symfony 6.4 / Symfony 7.0 readiness

Problem/Motivation

Symfony 6.4 will be released in November 2023 (https://symfony.com/releases/6.4).

Steps to reproduce

Proposed resolution

Start updating now to Symfony 6.4 to see what deprecations, if any, we need to fix.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet