T245482: Remove core fallbacks to global $wgUser [1.35] || NOTYETCREATED
See T242935: Deprecate core fallbacks to $wgUser for deprecations
Following hard deprecations in 1.35, some fallbacks should be removed.
Subtasks are for each fallback that should be removed. All deployed callers have already been updated, but ideally calls from non-deployed extensions will still be updated.