Consider making specVersion an integer with validations

Currently specVersion is a [string](https://github.com/CycloneDX/specification/blob/master/schema/bom-1.6.schema.json#L25). This is creating confusion when consuming tools treat this value as both string and integer.

Example:

https://github.com/CycloneDX/cyclonedx-maven-plugin/blob/925b04fdd74e4e412e1cc06d7fad9e7a102e329c/src/main/java/org/cyclonedx/maven/DefaultModelConverter.java#L236

https://github.com/CycloneDX/cyclonedx-maven-plugin/blob/925b04fdd74e4e412e1cc06d7fad9e7a102e329c/src/it/makeBom/verify.groovy#L11

https://github.com/DependencyTrack/dependency-track/blob/b40ea44864d006079d38a8d159c2d9d1c5fb04f7/src/main/java/org/dependencytrack/model/Vex.java#L131