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>

schema/cyclonedx/README.md

@@ -7,3 +7,12 @@ For this reason we've included a copy of all schemas needed to validate `syft` o
 to reference local copies of dependent schemas.
 
 You can get the latest schemas from the [CycloneDX specifications repo](https://github.com/CycloneDX/specification/tree/master/schema).
+
+When the spec version is bumped an approach to determining prior modifications is to compare the 
+prior spec version (e.g. if updating to 1.7, compare the files in this directory against the 1.6 
+equivalents). 
+
+One can also update the schemas and observe the errors in order to make the necessary updates. 
+At the time of writing, the cyclonedx.xsd needed modifications to link to the local spdx.xsd, 
+and also to changes the minOccurs for a license tag to 0. (The json schema does not require 
+modification for the generated file to lint properly, but can simply be copy/pasted).