Cluster export fails when local cluster name contains hyphens

# Issue report

## What version of MicroCeph are you using ?

tracking: squid/stable
installed: 19.2.1+snap74c0060321 (1582) 128MB held

## What are the steps to reproduce this issue ?

1. Bootstrap a MicroCeph cluster using a hostname that contains hyphens.
2. Attempt to export this cluster in order to enable Ceph replication.

## What happens (observed behaviour) ?

root@cluster-with-hyphens:~# microceph cluster export anotherclustername
Error: failed to fetch cluster state: cluster names can only have [a-z] or [0-9] characters: %!w()

**Note:** The issue is not related to the remote cluster name.
The validation fails due to the local cluster name (the one being exported).

## What were you expecting to happen ?

Since MicroCeph allows cluster names with hyphens during bootstrap and cluster add, it should also accept them during cluster export/cluster import, unless this is a strict upstream Ceph limitation (which does not seem to be the case).

## Additional information

The validation appears to originate from the regex defined in microceph/constants/constants.go

```
// ClusterNameRegex is the regex for validating cluster names (a-z0-9)
const ClusterNameRegex = "^[a-z0-9]+$"
```

This regex does not allow hyphens, which leads to the inconsistency.