[chore] Reduce vCPU count requirements on instances

[jplock]

We are asking to reserve 6 CPUs for nitro enclaves at https://github.com/aws-samples/sample-code-for-a-secure-vault-using-aws-nitro-enclaves/blob/main/vault_template.yml#L250

ACM for Nitro Enclaves requires 2 vCPUs to operate, 1 vCPU is required for the parent instance, leaving 1 vCPU available for the enclave application.

When launching a parent instance, choose an instance type that has enough vCPUs and memory for both the parent instance and the additional enclaves. If multi-threading is enabled, you must leave at least 2 vCPUs for the parent instance. If multi-threading is not enabled, you must leave at least 1 vCPU for the parent instance. For example, if multi-threading is enabled and you intend to run 4 enclaves with 4 vCPUs each, you must select an instance type that has at least 18 vCPUs (2 for the parent instance and 16 for the enclaves).
source

We should be able to reduce the minimum CPU count in the launch template from 8 to 4.

https://github.com/aws-samples/sample-code-for-a-secure-vault-using-aws-nitro-enclaves/blob/main/vault_template.yml#L634

[jplock]

I thought I could use the CoreCount and ThreadsPerCore properties in https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-ec2-launchtemplate-cpuoptions.html to disable multi-threading so we could support 4 vCPU machines (1 vCPU for parent, 2 vCPUs for Nitro ACM, 1 vCPU for Nitro Vault), but it became too complicated aligning to instance types.

[jplock]

Fixed by converting to Graviton in #194