Problem upgrading a Contrib extension requiring org.xwiki.contrib:authservice-backport-api

Cause: Module xwiki-platform-legacy-oldcore does not expose the same extension features as xwiki-platform-oldcore.

This is because the property `xwiki.extension.features` are not carried over to legacy modules.

In practice, this can cause issues for instances of XWiki that get upgraded and had the Authentication Backport contrib extension installed.