refactor: rename security-checks to scanners

[knqyf263]

Description

This PR renames --security-checks to --scanners for clarity. We leave the --security-checks flag available as well, so it is not a breaking change. The help output and document mention only --scanners, but users can specify --security-checks.

Before

$ trivy image --security-checks vuln debian:9

After

$ trivy image --scanners vuln debian:9

Checklist

• I've read the guidelines for contributing to this repository.
• I've followed the conventions in the PR title.
• I've added tests that prove my fix is effective or that my feature works.
• I've updated the documentation with the relevant information (if needed).
• I've added usage information (if the PR introduces new options)
• I've included a "before" and "after" example to the description (if the PR is a user interface change).

[knqyf263]

@itaysk Would you review it to check if it follows your suggestion?