v1.9.2: remove make audit; make sec is the single scanner entry point

Removes make audit; make sec is the single security-scan entry point.

- make audit only ran govulncheck, and make sec already runs it (govulncheck +
  semgrep, merged into sec.sarif). Keeping both was a redundant second way to run
  the same scanner, so make audit is gone. Run make sec instead. CI already used
  make sec, so the security gate is unchanged.