Bom validation in conflict with #573

[tmuehlbacher]

#573 makes it so that there may be two license objects one with the expression from the Cargo.toml package.license field and the other from the package.license-file found in the repo.

When using the Bom validation functionality, this causes this error

cyclonedx-rust-cargo/cyclonedx-bom/src/models/license.rs

Line 164 in 4b098ca

"Use either array of 'license' or a single 'expression', but not both.",

I am not sure which one of these should win and don't really have a preference.

As a real world example for a crate version where this happens, see https://github.com/owo-colors/owo-colors/blob/v3.5.0/Cargo.toml