scheduler: Use runner-first-visits

It was currently implemented as a cron-like service but a runner is better suited.

This unifies with the way this will be deployed in production (and matches an earlier discussion).

This requires a bump in swh.scheduler [1] to lift the runner. In the mean time, the tests will fail [2].

Refs. swh-scheduler#4687

[1] https://gitlab.softwareheritage.org/swh/devel/docker/-/merge_requests/new?merge_request%5Bsource_branch%5D=mr%2Fadapt-runner-first-visits

[2] Expectedly without a scheduler bump.

[2024-10-17T14:21:02.971Z] tests/test_save_bulk.py::test_save_bulk FAILED                           [ 70%]