XWiki Skins don't get the proper sheet for view & edit upon upgrades from 6.4.8

In 7.0, the sheet used for the skin pages has changed in order to implement XWIKI-11803 , from XWiki.XWikiSkinsSheet to SkinsCode.XWikiSkinsSheet.

However, upon upgrade from 6.4.8 to 10.11.9 (and to any version above 7.4, actually), the skin does not get the proper sheet associated to it upon upgrade, the old (inexistent) sheet remains associated to the skins class.
The result is that the view & edit UI don't correspond to the expected UI, the one implemented for XWIKI-11803 - view shows nothing while edit is editing the document content.

The cause for this is the removal of the XWikiSkins class from the skins application, by https://github.com/xwiki/xwiki-platform/commit/864150b7dbf9bed6280bf7ac8826c87888e10563 along with an improper initialization of the skins class in https://github.com/xwiki/xwiki-platform/blob/xwiki-platform-10.11.9/xwiki-platform-core/xwiki-platform-oldcore/src/main/java/com/xpn/xwiki/internal/mandatory/XWikiSkinsDocumentInitializer.java#L83 which associates the sheet to the skin class only if there is none other associated already: as the old one from 6.4.8 is already associated, this code does not set the proper sheet.