feat: update syft to generate cyclone-dx 1.6 by default (#2978)

- Resolves #2974
- add detailed instructions re: updating schemas (a necessary task
  when a new CycloneDX spec version becomes available).
- The DefaultVersion constant has been updated to "1.6" -- it's not
  clear to me how this is used at this time (it may be redundant given
  other code), but effectively unless a specific spec version is
  configured, `syft` will emit the "most recent" spec version available
  for cyclonedx. Users who wish to pin back to a "older" specVersion
  (e.g. to preserve compatibilty with utilities that have not yet bumped
  to latest) can either set this in a syft config file or pass a
  name@spec_version pair to the output flag (e.g. `-o
  cyclonedx-json@1.5=some-1.5-spec-bom.cdx.json`)
- Regenerate relevant .golden files (there seems to be a way to do this
  via flags, but I couldn't quite figure out the right set to pass
  correctly, esp. since (as a relative go novice) I found it difficult
  to run just a single test file. I ended up "brute-forcing it" by
  changing the *updateSnapshot val to "true" and running it in Goland.
  A brief comment giving an example of regenerating fixtures usage would
  be helpful.

Signed-off-by: Rajan Agaskar <ragaskar@gmail.com>

syft/format/internal/cyclonedxutil/encoder.go

@@ -9,7 +9,7 @@ import (
 	"github.com/anchore/syft/syft/sbom"
 )
 
-const DefaultVersion = "1.5"
+const DefaultVersion = "1.6"
 
 type Encoder struct {
 	version cyclonedx.SpecVersion

syft/format/internal/cyclonedxutil/versions.go

@@ -19,6 +19,7 @@ func SupportedVersions(id sbom.FormatID) []string {
 		"1.3",
 		"1.4",
 		"1.5",
+		"1.6",
 	}
 
 	if id != JSONFormatID {
@@ -43,6 +44,8 @@ func SpecVersionFromString(v string) (cyclonedx.SpecVersion, error) {
 		return cyclonedx.SpecVersion1_4, nil
 	case "1.5":
 		return cyclonedx.SpecVersion1_5, nil
+	case "1.6":
+		return cyclonedx.SpecVersion1_6, nil
 	}
 	return -1, fmt.Errorf("unsupported CycloneDX version %q", v)
 }
@@ -61,6 +64,8 @@ func VersionFromSpecVersion(spec cyclonedx.SpecVersion) string {
 		return "1.4"
 	case cyclonedx.SpecVersion1_5:
 		return "1.5"
+	case cyclonedx.SpecVersion1_6:
+		return "1.6"
 	}
 	return ""
 }