Rename Backend and Frontend code related to sast_container into container_scanning

Problem to solve

Following https://gitlab.com/gitlab-org/gitlab-ee/issues/5017 we still need to update the code to have a consistent naming of the container scanning feature.

Intended users

Further details

Proposal

Permissions and Security

Documentation

Testing

What does success look like, and how can we measure that?

What is the type of buyer?

Links / references