Update SQLAlchemy to 2.x

[SamR1]

see https://docs.sqlalchemy.org/en/20/changelog/migration_20.html

[SamR1]

Flask-SQLAlchemy 3.1.0 (released 2023-09-11) adds support for the SQLAlchemy 2.x.

Blog post about migration: https://blog.pamelafox.org/2023/09/using-sqlalchemy-20-in-flask.html

[SamR1]

Since merge on add-social-features branch, tests are very slow on CI (and jobs are killed).
It seems to affect only CI (no problem locally).

=> move Flask-SQLAchemy/SQLAlchemy update with/after social features to investigate and avoid delaying fixes release.