[Bug/_MungeManager]: incorrect service name for MUNGE when using _AptManager

[NucciTheBoss]

As part of integration testing for charmed-hpc/slurm-charms#35, turns out we have the incorrect service name for MUNGE if using the _AptManager implementation. The service name is munge, not munged:

unit-slurmctld-0: 09:27:19 ERROR unit.slurmctld/0.juju-log command ['systemctl', 'reload-or-restart', 'munged'] failed with message Failed to reload-or-restart munged.service: Unit munged.service not found.

unit-slurmctld-0: 09:27:19 ERROR unit.slurmctld/0.juju-log command ['systemctl', 'reload-or-restart', 'munged'] failed. stderr:
Failed to reload-or-restart munged.service: Unit munged.service not found.

This issue wasn't caught because the integration tests don't try to start or stop the MUNGE service, but the fix is relatively easy as the _ServiceType enum will need to have the service name changed to munge:

hpc-libs/lib/charms/hpc_libs/v0/slurm_ops.py

Lines 177 to 180 in fda80b3

	class _ServiceType(Enum):
	"""Type of Slurm service to manage."""
	MUNGED = "munged"

The additional work that will be needed here is that we'll need to change the service name on the Slurm snap to munge instead of munged. We'll just need to make sure that the service name munge doesn't clash with the /snap/bin/munge binary on the snap package.