Move to using SAST_ANALYZER_IMAGE_TAG variable for SAST
Summary
SAST_MAJOR_VERSION was recently added to the SAST.gitlab-ci.yml template
We should move to using SAST_ANALYZER_IMAGE_TAG instead as it is already documented and does what we need. Please see #32787 (comment 239365902) for more background.
Improvements
This will get rid of an undocumented variable without any change to functionality.
Risks
This would be limited to the SAST analyzer, and more specifically, the variable we are removing is only used for the Docker-in-Docker-less version of SAST, which is not currently in widespread use. The use of the Docker-in-Docker version is determined by the SAST_DISABLE_DIND variable in .gitlab-ci.yml, so worst case scenario would be a customer had to go back to the DinD version of SAST.