build: add support for Connexion 3+

[uniqueg]

Problem

To ensure that FOCA will be able to benefit from security and feature updates of the archetype's core dependencies, sooner or later it will be necessary to upgrade to Connexion 3, especially because Connexion 2 is (and likely won't be) compatible with Flask 3 and Werkzeug 3. Moreover, issues #81 and #209 will be much easier to address after the migration.

Unfortunatley, the migration will not be a piece of cake:

Connexion 3 fundamentally changes how Connexion is designed and implemented, and how it fits into the wider Python API ecosystem. We adopted the ASGI interface, which makes Connexion both modular and well-integrated with most modern Python API tooling.