Use parent.version instead of project.version

In a provider-specific release, the provider pom versions (i.e., project.version) will different from the core pom version (unit-core/pom.xml's project.version). I don't know any direct way to access the project version of a peer module, but we can use the parent.version in this case, because the root pom is supposed to follow the version number of the core module.

This only matters for releases that include provider specific tags. In all other cases, the project.version and parent.version are the same value.

See failing pipeline in the release branch for the issue: e52ab126. The unit-aws artifact was updated to 0.28.1, while unit-core was still on 0.28.0. Updating unit-aws to a non-existent core version resulted in compilation to fail.