[4.9] Backport of "unknown instance type" error for supported m6i.xlarge instance

Original bug description:
AWS instance m6i.xlarge is supported [1], but it does not appear on ec2_instance_types.go [2] and the following errors show up when it is used (hidden information marked with "XXX"):

$ oc logs -l k8s-app=controller -n openshift-machine-api -c machine-controller
[...]
I0112 14:51:10.117191       1 controller.go:59] controllers/MachineSet "msg"="Reconciling" "machineset"="XXX" "namespace"="openshift-machine-api"
E0112 14:51:10.117604       1 controller.go:115] Unable to set scale from zero annotations: unknown instance type: %sm6i.xlarge
E0112 14:51:10.117622       1 controller.go:116] Autoscaling from zero will not work. To fix this, manually populate machine annotations for your instance type: %v[machine.openshift.io/vCPU machine.openshift.io/memoryMb machine.openshift.io/GPU]
I0112 14:51:10.117792       1 recorder.go:104] controller-runtime/manager/events "msg"="Warning"  "message"="Failed to set autoscaling from zero annotations, instance type unknown" "object"={"kind":"MachineSet","namespace":"openshift-machine-api","name":"XXX","uid":"XXX","apiVersion":"machine.openshift.io/v1beta1","resourceVersion":"121131434"} "reason"="FailedUpdate"

4.9

Always

Steps to Reproduce:
1. Create a MachineSet with m6i.xlarge instances.
2. Check the logs of machine-controller container.
3. 

Error messages show up, although no impact on the cluster.

No error messages because it is a supported instance.

[1] https://docs.openshift.com/container-platform/4.9/installing/installing_aws/installing-aws-customizations.html#installation-supported-aws-machine-types_installing-aws-customizations
[2] https://github.com/openshift-cherrypick-robot/cluster-api-provider-aws/blob/2f3f7442ef525c7eaa2177d1903cebf219a2c3fc/pkg/actuators/machineset/ec2_instance_types.go