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/cyclonedxjson/test-fixtures/snapshot/TestCycloneDxImageEncoder.golden

@@ -1,7 +1,7 @@
 {
-  "$schema": "http://cyclonedx.org/schema/bom-1.5.schema.json",
+  "$schema": "http://cyclonedx.org/schema/bom-1.6.schema.json",
   "bomFormat": "CycloneDX",
-  "specVersion": "1.5",
+  "specVersion": "1.6",
   "serialNumber": "urn:uuid:redacted",
   "version": 1,
   "metadata": {