Update variables of an existing .gitlab-ci.yml

In Write variables to a new .gitlab-ci.yml, we added the ability to write a few supported SAST variables to a new .gitlab-ci.yml file.

As a next iteration, we need to support updating supported SAST variables in an existing .gitlab-ci.yml file. This should also account for removing supported SAST variables that are no longer set.

Implementation Plan

• Add logic for updating a .gitlab-ci.yml
• Add specs