oci_core_subnet cidr_block change triggers an update when a replace is required

[slevey]

Changing an oci_core_subnet cidr_block attribute will trigger an update operation. An update assumes that the new cidr_block is a superset of the the original cidr_block.

For situations where the new cidr_block is not a superset, the correct action would be to perform a replace.

The below error occurs when changing the cidr_block from 10.0.7.0/25 to 10.0.0.128/25 Error Message: The requested cidrBlock 100.26.6.128/25 is invalid: cidrBlock does not have the original network address: 100.26.7.0.