chore: replace deprecated '--slow' with '--parallel'

[henkka]

Relates to aquasecurity/trivy#5572 where --slow was deprecated in the trivy project.

With current implementation, each scan job logs the following warning:

WARN    "--slow" is deprecated. Use "--parallel 1" instead.

This pull request modifies trivy-operator to use the --parallel 1 instead of the deprecated --slow flag, while keeping the trivy-operator's own API backward compatible (ie. call it slow but under the hood convert it to parallel)

[CLAassistant]

All committers have signed the CLA.

[afdesk]

maybe we should support backward compatibility with older tags too
--slow was deprecated in v0.53.0 aquasecurity/trivy#7061

[henkka]

Thank you for the quick turnaround! Fixed with fa22696

[afdesk]

@henkka thanks for your contribution
i left a small comment

[afdesk]

@simar7 should we change a flag --slow to --parallel in trivy-operator too

trivy-operator/pkg/plugins/trivy/flags.go

Lines 22 to 35 in fb9add8

	func Slow(c Config) string {
	tag, err := c.GetImageTag()
	if err != nil {
	return ""
	}
	// support backward compatibility with older tags
	if compareTagVersion(tag, "< 0.35.0") {
	return ""
	}
	if c.GetSlow() {
	return "--slow"
	}
	return ""
	}