Fix the version of AWS CloudFormation templates used to provision platform external clusters on AWS.

Currently[1] the version is set to a custom version from Github org openshift-splat-team, we should migrate it to the final repo* and version:

*The repository has been renamed from "labs" to "installer-labs".

[1] https://github.com/openshift/release/blob/master/ci-operator/step-registry/platform-external/cluster/aws/install/platform-external-cluster-aws-install-commands.sh#L60-L62