Bug 1725524: types/aws/default: move ap-northeast-2 to m5 instance class

AWS allows m4 class for reserved instances in all AZs for ap-northeast-2

```console
aws --region ap-northeast-2 ec2 describe-reserved-instances-offerings --instance-tenancy default --instance-type m4.large --product-description 'Linux/UNIX' --filters Name=scope,Values='Availability Zone' | jq -r '[.ReservedInstancesOfferings[].AvailabilityZone] | sort | unique[]'
ap-northeast-2a
ap-northeast-2b
ap-northeast-2c
```

But the on-demand instances in ap-norteast-2b was failing with error
```
Unsupported: Your requested instance type (m4.xlarge) is not supported in your requested Availability Zone (ap-northeast-2b). Please retry your request by not specifying an Availability Zone or choosing ap-northeast-2a, ap-northeast-2c.
```

So moving to m5 class for ap-northeast-2 should allow creating control plane instances in all Zones. (the default configuration)

pkg/types/aws/defaults/platform.go

@@ -6,11 +6,12 @@ import (
 
 var (
 	defaultMachineClass = map[string]string{
-		"ap-east-1":     "m5",
-		"eu-north-1":    "m5",
-		"eu-west-3":     "m5",
-		"us-gov-east-1": "m5",
-		"us-west-2":     "m5",
+		"ap-east-1":      "m5",
+		"ap-northeast-2": "m5",
+		"eu-north-1":     "m5",
+		"eu-west-3":      "m5",
+		"us-gov-east-1":  "m5",
+		"us-west-2":      "m5",
 	}
 )