Doc update: state that Vault is preferred to keys over ceph-mon

[Ponnuvel Palaniyappan]

In recent discussions, it's been recommended that Vault should be used instead of ceph-mon for storing the keys.

However, the documentation [0] suggests either could be used:
```
The ceph-osd charm supports encryption for OSD volumes that are backed by block devices. To use Ceph's native key management framework, available since Ceph Jewel, set option osd-encrypt for the ceph-osd charm:

    ceph-osd:
      options:
        osd-encrypt: True
Here, dm-crypt keys are stored in the MON sub-cluster.

Alternatively, since Ceph Luminous, encryption keys can be stored in Vault, which is deployed and initialised via the vault charm. Set options osd-encrypt and osd-encrypt-keymanager for the ceph-osd charm:

    ceph-osd:
      options:
        osd-encrypt: True
        osd-encrypt-keymanager: vault
```

So this needs to properly documented with relevant caveats: what's preferred, what should be done for existing/old deployments that use ceph-mon, how to handle upgrades, etc.

[0] https://jaas.ai/ceph-osd