COU should check supported Juju version and raise proper exeption

[rgildein]

Right now running COU on 2.9.x failed with not obvious message, which tell user that current Juju is not supported.

For example, running COU against 2.9.35 you will see this output:

$ cou plan  
Full execution log: '/home/rgildein/.local/share/cou/log/cou-20240702172421.log'  
Connecting to 'admin/test' model... ✖
The connection was lost.
Check your connection or increase the timeout.
Default timeout is 10s; to increase to 60s for example:
$ COU_TIMEOUT=60 cou plan

After checking logs, you will see proper error:

$  cat /home/rgildein/.local/share/cou/log/cou-20240702172421.log
...
juju.errors.JujuAPIError: client incompatible with server 2.9.35 

I believe that COU should check the controller version and raise proper error with proper message. e.g.

$ cou plan  
Full execution log: '/home/rgildein/.local/share/cou/log/cou-20240702172421.log'  
Connecting to 'admin/test' model... ✖
Cliet version 2.9.35  is not supported, please upgrade to Juju 3.X