Deprecate CLAIR_OUTPUT in Container Scanning

The purpose of this issue is to deprecate the CLAIR_OUTPUT variable used in container scanning and to replace it with a variable that better describes the actual behaviour, such as SEVERITY_LEVEL_THRESHOLD

TODO:

• Update container scanning docs to deprecate CLAIR_OUTPUT and add SEVERITY_LEVEL_THRESHOLD
• Update GitLab klar README
• Update GitLab klar source to support both CLAIR_OUTPUT and SEVERITY_LEVEL_THRESHOLD