Fix deploy to root image

Fix a regression introduced by !2502 (merged)

The images weren't being deployed to the root image on either repository because the jobs override the rules section, which was being used to declare a DEFAULT_IMAGE variable.

I've adjusted the logic to replace the rules declaration with a global DEFAULT_SCANNER variable.

This also bumps the version to 4.2.1 so we can cut a new release and have up-to-date images.

Tests

Tests were performed in a forked repository at https://gitlab.com/thiagocsf-group/container-scanning

• tag version: [trivy]: push to root image
• tag version: [grype]: does not push to root image
• major: [trivy]: push to root image
• major: [grype]: does not push to root image
• latest: [trivy]: push to root image
• latest: [grype]: does not push to root image

Pipeline with the same jobs as above but with the CS_DEPLOY_* variables populated so that the second repository push is activated (failures are expected since the credentials are bogus).