Trigger `lock.yml` workflow through dependabot

[redeboer]

Now that Dependabot supports uv and supports Julia we can use it as a trigger for upgrading lock files. This would largely fix #346.

For now, we solve #346 for public repositories by triggering the lock upgrade with pre-commit.ci autoupdate, but this is not available for private repositories and also does not support a semiannual schedule. Dependabot does work for private repositories, supports more schedules, and is integrated into the GitHub dependency graph interface. Once available, we can add more triggers, like for Pixi and for pre-commit itself.

Note that Dependabot does not update dependencies once the PR has been made. For this reason, the lock.yml workflow itself is not yet reduntant.