Upgrade manager to Django 5.1

[rgaudin]

There are a few alerts on Django (that does not concern this codebase).

Based on release notes upgrading implies at least:

• upgrading mariadb to 10.5 or newer (we use 10.4 for which Django 5.1 drops support)
• replace make_random_password()
• ensure we never pass encoded JSON literal to JSONField

[end-9214]

can i work on this?

[rgaudin]

Most of this is upgrading our database which you can't do obviously so there's little value in assigning this to you. Please look for good-first-issue label