Investigate generation of SPDX v3.0 documents for Qt build SBOM

Qt 6.8 generates SPDX v2.3 documents for the build SBOM.

The latest version is currently v3.0.

We should investigate what it would take to generate the version.

Current roadblocks for that are:

• v3.0 doesn't seem to support INI-style key:value SPDX documents, but only JSON.
  For v2.3 we generate INI-style documents, and convert them to JSON via external tooling.
  CMake doesn't have a good facility for generating complex JSON documents, so it will probably be awkward to reimplement that.

• Tooling to check the validity of v3.0 documents is not there yet. The ones we currently use in the CI are spdx_tools, sbom2doc, sbomaudit, ntia_performance_checker.

• The semantic structure of the documents has change considerably between v2.3 and v3.0 so it will need investigation on how to adapt that.