The API key that is underprivileged for MSS and Premium
is still considered as valid because the API can still authenticate the key
and the key can be used reporting subscriber stats etc.
I've check places where we use the property and in all cases we use
to distinguish if the plugin uses a subscription plan (free or paid) or no plan.
[MAILPOET-5191]
The values we have in key state might be missing or outdated.
We need to check also if the key check on MSS returned error
about access being restricted due email volume limit.
[MAILPOET-5191]
Currently the starter plan has no access to premium so the plugin is
not able to fetch email volume limit using premium key endpoint.
Also the data are related to sending so it makes more sense to read those
from MSS key state.
[MAILPOET-5191]
