refactor(trivy): enforce usage of Trivy environment variables

- $CNB_TRIVY_ADDR removed in favor of $TRIVY_SERVER
- $CNB_TRIVY_SECURITY_LEVEL_THRESHOLD removed in favor of $TRIVY_SEVERITY
- $CNB_TRIVY_DB_REPOSITORY removed in favor of $TRIVY_DB_REPOSITORY
- $CNB_TRIVY_JAVA_DB_REPOSITORY removed in favor of $TRIVY_JAVA_DB_REPOSITORY

BREAKING CHANGE: 4 Trivy configuration params removed in favor of the native Trivy environment variables