Extension Updater should let global admins choose whether to compute the upgrade plan for the entire farm or just for the main wiki

Currently, the Extension Updater triggers the upgrade plan automatically when you access the administration section if there is no previous plan computed or the previous plan is outdated. On subwikis it computes the upgrade plan only for those subwikis. On the main wiki, it computes the upgrade plan based on the upgrade mode which we recently dropped. So after XWIKI-10186 removed the use of the upgrade mode now global admins can't choose any more between getting the upgrade plan for the main wiki only (+ global extensions) and getting it for the entire farm.

On a farm like xwiki.org which is managed by a single entity we want to get the upgrade plan for the entire farm. On a farm like myxwiki.org or on a cloud environment where each wiki is managed by a different entity, we don't want to compute the upgrade plan for the entire farm (which will take a while) but only for the main wiki and the global extensions.

A solution is to stop triggering the upgrade plan automatically when accessing the Extension Updater section and add a button group instead that the user can use to choose the scope of the upgrade plan.