Document known Trac limitations

While I'm looking for potential cleanups in the repo, I learned that

* Trac doesn't support Python 3.12, so the latest Python version we can use is 3.11: https://github.com/django/code.djangoproject.com/pull/219#issuecomment-2452482455
* Trac only support `psycopg2`, we can't switch to `psycopg` - the newer versions: https://github.com/django/code.djangoproject.com/pull/223#issuecomment-2458100629

I think both are important issues that should be documented in a visible way. This may help the volunteers working on the repo and also contribute to the move-out of Trac discussions.