Default Lambda smoke testing to use known stable stack versions

[axw]

https://github.com/elastic/apm-aws-lambda/pull/296/files#r971404965

We could either do what is described in the comment there, or perhaps update ec_deployment to default to using the most recently verified snapshot version (i.e. what's defined in docker-compose.yml).

[kruskall]

The terraform script is defaulting to the latest stable version due to #320

I think this can be closed