[TRACKING] Complete support for CycloneDX v1.6

see https://github.com/CycloneDX/specification/releases/tag/1.6
and https://github.com/CycloneDX/specification/releases/tag/1.6.1

The following items are not currently supported for CycloneDX v1.6 (as of release 7.6.x):
- [ ] definitions
- partially addressed by #701
- partially addressed by #713
- TODO: `definition.Standard.signature`
- [ ] `dependencies.provides`
- via https://github.com/CycloneDX/cyclonedx-python-lib/pull/735
- [ ] `bom-ref` properties here and there
- issue #799
- possible fix: #859
- [ ] declarations
- to be continued

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this library is a community effort.
if you find a feature that you need is missing, feel free to donate/contribute the missing feature.
- see #633