`controller-url` config value set on controller charm is badly formatted

[Jordan Barrett]

The `controller-url` config key is set on the controller charm. This value is passed to apps related to the controller charm (juju-dashboard, prometheus, etc) so that they can locate the controller.

However, on some clouds (e.g. microk8s), this value is incorrectly formatted.
```
$ juju bootstrap microk8s c
$ juju switch controller
$ juju config controller controller-url
wss://[10.152.183.158:0]:17070/api
```

Tested on:
- Juju version version: 3.1-d48908d
- microk8s v1.26.7
- Ubuntu 20.04.6 LTS

[Jordan Barrett (barrettj12)]

On this note, we shouldn't be providing the full URL with 'wss://' and '/api'. In many cases, consumers just have to strip this part to get what they actually want. See
https://github.com/canonical/juju-dashboard-charm/blob/eeb083d/machine-charm/src/charm.py#L86
https://github.com/canonical/juju-dashboard-charm/blob/eeb083d/k8s-charm/lib/charms/juju_dashboard/v0/juju_dashboard.py#L81-L88

We should deprecate `controller-url` and instead provide just the controller IP and port, without the scheme or API endpoint (either as two separate keys, or as one key 'host:port').