Fix 'Namespace' object has no attribute 'upgradeable_to'

[bitoku]

Which issue this PR addresses:

Fixes N/A

What this PR does / why we need it:

When I try to create a cluster with --client-id and --client-secret, it failed with the error 'Namespace' object has no attribute 'upgradeable_to'.

--upgradeable-to doesn't exist for create subcommand, but --client-id and --client-secret options have a validation to validate namespace.upgradeable_to.
So the validator tries to access namespace.upgradeable_to and it failed.

This PR makes the validator aware of if it is create or update, and don't validate upgradeable_to when it is create.

This is the error I got (with --debug)

❯ az aro create \
--resource-group $MY_RESOURCEGROUP \
--name $MY_CLUSTER \
--vnet aro-vnet \
--master-subnet master-subnet \
--worker-subnet worker-subnet \
--pull-secret @/Users/atokubi/Downloads/pull-secret.txt \
--version 4.15.35 \
--cluster-resource-group aro-$MY_CLUSTER \
--client-id *** \
--client-secret ***
...
cli.azure.cli.core.azclierror: Traceback (most recent call last):
  File "/usr/local/Cellar/azure-cli/2.59.0/libexec/lib/python3.11/site-packages/knack/invocation.py", line 113, in _validation
    self._validate_arg_level(parsed_ns)
  File "/usr/local/Cellar/azure-cli/2.59.0/libexec/lib/python3.11/site-packages/azure/cli/core/commands/__init__.py", line 894, in _validate_arg_level
    validator(**self._build_kwargs(validator, ns))
  File "/Users/atokubi/ARO-RP/python/az/aro/azext_aro/_validators.py", line 55, in validate_client_id
    if namespace.upgradeable_to is not None:
       ^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'Namespace' object has no attribute 'upgradeable_to'

Test plan for issue:

Cluster creation with az cli.

Is there any documentation that needs to be updated for this PR?

N/A

How do you know this will function as expected in production?

cluster creation with az cli.

[bitoku]

@slawande2 Can you PTAL and confirm this change doesn't break your change #3844 ?