[server] replace repository's tasks manager by a scheduler Repository() does not accept anymore a 'tasks_manager' argument but rather a 'scheduler' argument which is expected to be an instance of sched.scheduler class. The drop the _tasks_manager attribute of the repository and adjust all internal usages of it. In particular, in the 'repo_stats' service we do not export 'looping_tasks' statistics anymore as there's no way to retrieve this anymore from a web instance. Closes #17057223.

Twisted < 16.0.0
requests
webtest
cubicweb-blog
cubicweb-file
cubicweb-tag