Add support for the cflinuxfs4 stack

We have added an experimental ops file to cf-deployment which enables the cflinuxfs4 stack. Now CATs should support the new stack. Currently cflinuxfs3 is hard-coded in the config validation:

cf-acceptance-tests/helpers/config/config_struct.go

Line 710 in b606d93

for _, stack := range config.GetStacks() {

We are already running CATs with cflinuxfs4 in the cf-deployment pipeline. The job is currently green:
https://app-runtime-deployments.ci.cloudfoundry.org/teams/main/pipelines/cf-deployment/jobs/experimental-cats-cflinuxfs4

It doesn't test the buildpacks with the new stack, however. This has to be implemented here:
https://github.com/cloudfoundry/cf-acceptance-tests/blob/develop/detect/buildpacks.go

Released new CATs version: https://github.com/cloudfoundry/cf-acceptance-tests/releases/tag/v9.2.0
This version now supports "cflinuxfs4" as stack and the buildpack detect test suite tests all configured stacks.