docs: Configuring cluster network misses a step to restart microceph

Configuring cluster network documentation misses a step to restart microceph.

After setting the cluster network, it is also required to restart microceph services.

To reproduce:

Check the network config list:

```
$ sudo microceph cluster config list
+---+-----------------------------+----------------+
| # | KEY | VALUE |
+---+-----------------------------+----------------+
| 0 | cluster_network | 10.20.116.0/24 |
+---+-----------------------------+----------------+
| 1 | osd_pool_default_crush_rule | 3 |
+---+-----------------------------+----------------+
| 2 | public_network | 10.20.116.0/24 |
+---+-----------------------------+----------------+
| 3 | rbd_default_features | 63 |
+---+-----------------------------+----------------+
```

Update the cluster network:

```
$ sudo microceph cluster config set cluster_network 10.20.121.0/24
```

Check that it has been updated:

```
$ sudo microceph cluster config list
+---+-----------------------------+----------------+
| # | KEY | VALUE |
+---+-----------------------------+----------------+
| 0 | cluster_network | 10.20.121.0/24 |
+---+-----------------------------+----------------+
| 1 | osd_pool_default_crush_rule | 3 |
+---+-----------------------------+----------------+
| 2 | public_network | 10.20.116.0/24 |
+---+-----------------------------+----------------+
| 3 | rbd_default_features | 63 |
+---+-----------------------------+----------------+
```

Ceph OSDs still use the old network for cluster communication:

```
$ sudo ss -tulpn | grep osd
tcp LISTEN 0 512 10.20.116.90:6802 0.0.0.0:* users:(("ceph-osd",pid=91764,fd=19))
tcp LISTEN 0 512 10.20.116.90:6803 0.0.0.0:* users:(("ceph-osd",pid=91764,fd=20))
tcp LISTEN 0 512 10.20.116.90:6806 0.0.0.0:* users:(("ceph-osd",pid=91764,fd=23))
tcp LISTEN 0 512 10.20.116.90:6807 0.0.0.0:* users:(("ceph-osd",pid=91764,fd=24))
tcp LISTEN 0 512 10.20.116.90:6804 0.0.0.0:* users:(("ceph-osd",pid=91764,fd=21))
tcp LISTEN 0 512 10.20.116.90:6805 0.0.0.0:* users:(("ceph-osd",pid=91764,fd=22))
tcp LISTEN 0 512 10.20.116.90:6808 0.0.0.0:* users:(("ceph-osd",pid=91764,fd=25))
tcp LISTEN 0 512 10.20.116.90:6809 0.0.0.0:* users:(("ceph-osd",pid=91764,fd=26))
```

Restart microceph:

```
$ sudo snap restart microceph
```

Check again:

```
$ sudo ss -tulpn | grep osd
tcp LISTEN 0 512 10.20.121.137:6800 0.0.0.0:* users:(("ceph-osd",pid=894935,fd=21))
tcp LISTEN 0 512 10.20.121.137:6801 0.0.0.0:* users:(("ceph-osd",pid=894935,fd=22))
tcp LISTEN 0 512 10.20.121.137:6802 0.0.0.0:* users:(("ceph-osd",pid=894935,fd=25))
tcp LISTEN 0 512 10.20.121.137:6803 0.0.0.0:* users:(("ceph-osd",pid=894935,fd=26))
tcp LISTEN 0 512 10.20.116.90:6806 0.0.0.0:* users:(("ceph-osd",pid=894935,fd=23))
tcp LISTEN 0 512 10.20.116.90:6807 0.0.0.0:* users:(("ceph-osd",pid=894935,fd=24))
tcp LISTEN 0 512 10.20.116.90:6804 0.0.0.0:* users:(("ceph-osd",pid=894935,fd=19))
tcp LISTEN 0 512 10.20.116.90:6805 0.0.0.0:* users:(("ceph-osd",pid=894935,fd=20))
```

---
*Reported from: https://canonical.com/ceph/docs/latest/snap/how-to/configure-network-keys/*

Read the cluster network configuration guide at `https://canonical.com/ceph/docs/latest/snap/how-to/configure-network-keys/`. Add the missing instruction to restart MicroCeph services after changing the cluster network. Done when the guide includes the restart step and its verification context.

Written by the indexing model from the issue text.