Store the previous release as a property in the parent

[laeubi]

Currently we only have the information about the current release name but at some places the version of the previous one would be useful.

This adds a new property 'previousReleaseVersion' to contain the necessary information and is updated by the prepare-release workflow.

This also includes

• https://github.com/eclipse-platform/eclipse.platform.releng.aggregator/pull/2575

what probably should be merged first, also the workflow needs to be tested one to produce the desired results.