Compatibility with latest Big Bang version

As a user of this Terraform module, I want it to be compatible with the latest version of Big Bang, so that I can use it to deploy current Big Bang

Notes:

• BB 1.14.1's ingressgateway is named public-ingressgateway now instead of istio-ingressgateway
• There are breaking changes between the version of Flux that gets deployed now and the version that Big Bang v1.14.1 expects