Use latest/stable branch of Slurm snap

[NucciTheBoss]

Currently we're using the latest/candidate version of the Slurm snap in slurm_ops. This is fine for the initial development of the Slurm snap, but eventually we need to switch over to using the latest/stable channel as that is guaranteed to have more rigorous testing. latest/candidate is what is directly published from the Slurm snap CI actions which means it has a higher chance of bugs slipping through.

We'll also need to figure out how to properly handle channels, and how we can possibly allow admins to configure channels within the Slurm charms.