Avoid creating subnets in disallowed Availability Zone IDs

[cPu1]

Some regions do not support creating subnets in all availability zone IDs, as documented in Subnet requirements for clusters. If eksctl attempts to create a subnet in one of the aforementioned zone IDs, subnet creation will fail with an error.

The aim should be for a generic solution that does not require hardcoding the zone IDs from that list to avoid having to keep this list up-to-date.