[diracx-tasks] `diracx-task-run` should be `diracx-tasks`

[chaen]

The name was poorly chosen as it is not just to run a task.
This involves renaming the scripts, updating the doc, and updating the chart.

Needs to be done in 3 PRs:

• first in diracx, you need to support both script names
• once merged, then in diracx-charts, you drop the old script name
• once merged, you drop the old script name in diracx

You don't need to understand diracx-charts or helm here, you just need to find the diracx-task-run mention within it and replace it with the new name.

what's diracx-task-run exactly? Potentially no documentation about that (see admin guide how-to/tasks in the documentation)