Link to Purl Version Specification does not work

[weichslgartner]

The link to the purl version range defined in https://github.com/CycloneDX/specification/blob/8af880d5f2ba0a107de88a920a76cedd5ba75083/schema/bom-1.5.xsd#L3647 (https://github.com/package-url/purl-spec/VERSION-RANGE-SPEC.rst) does not work at the moment.

Currently the Version-Range Spec is not merged to main but only available in a different branch: https://github.com/package-url/purl-spec/blob/version-range-spec/VERSION-RANGE-SPEC.rst

Once the spec is merged the problem should resolve by itself, but in the meantime it is a dead link.

[jkowalleck]

duplicates #311, #399 & related to #187