Update demos to use Maven 4.0.0-rc-4.

At the same time, cleanup plexus-utils usage:
* on top we use 3.6.0
* Wagon uses it but at runtime Resolver/Maven will provide it
* Tools (docgen) uses it, but is happy with 3.6.0
* Demo snippets aligned with updated Maven 4 version

Unsure what would be the best here, as demos must to override
management from top level POM.

cstamas marked this pull request as ready for review

cstamas deleted the plexus-utils-cleanup branch