Remove 'sast_container' value from GITLAB_FEATURES environment variable

In 11.5 we deprecated sast_container value in favor of container_scanning as the right name of the feature exposed via GITLAB_FEATURES environment variable.

Now we could evaluate to remove the old value from the variable. The usage is probably very low, as it was not widely used. We should also check that nothing is still using it internally (e.g., Auto DevOps, gitlab-ce repo, etc.).

• Check that the sast_container name is not used anymore internally (e.g., Auto DevOps, gitlab-ce repo, etc.)
• Remove the sast_container value from GITLAB_FEATURES environment variable
• [ ] Update documentation to state the old name has been removed There is no more documentation mentioning this.