isSuperAdmin is implemented in many different ways

Across our codebase, we have a dozen way to check if a user is superadmin

• com.xpn.xwiki.internal.plugin.rightsmanager.UserIterator#isSuperAdmi
• com.xpn.xwiki.user.api.XWikiRightService#isSuperAdmin
• com.xpn.xwiki.user.impl.xwiki.AbstractXWikiAuthService#isSuperAdmin
• com.xpn.xwiki.user.impl.xwiki.XWikiRightServiceImpl#isSuperAdmin
• com.xpn.xwiki.internal.template.InternalTemplateManager.FilesystemTemplateContent#isPrivileged
• com.xpn.xwiki.user.impl.xwiki.XWikiRightServiceImpl#isSuperAdmin
• com.xpn.xwiki.user.impl.xwiki.XWikiRightServiceImpl#isSuperAdminOrProgramming
• org.xwiki.test.checker.internal.ProgrammingRightCheckerAuthorizationManager#check(org.xwiki.model.reference.DocumentReference, com.xpn.xwiki.doc.XWikiDocument)
• org.xwiki.user.internal.document.AbstractUserReferenceResolver#isSuperAdmin

It would be better to have a single source of truth, especially as some implementations might ne be aligned with each other