Doesn't do much for the client; mostly a safety for osu-web's sake,
as without the change it could theoretically fail to validate the mod
properly in multiplayer contexts.
The mod generally will only be present on scores imported from stable.
As such, it's probably ok to mark it as such.
The primary reason for this change is to address #24436 (Score V2 being
visible on beatmap overlay leaderboard mod selector).
There is one possibly-unintended consequence of this change, namely that
the results screen uses `UserPlayable` to determine as to whether
animations should be played back, with the intention of turning off the
animation playback for autoplay scores specifically. Therefore, turning
off this flag will mean that the results screen animations will not play
out for Score V2 scores - but I tend to consider this as either largely
unimportant, or something that should be fixed in some other way
(possibly by checking against the autoplay mod directly).
Other usages of `UserPlayable` are either innocuous, or straight-up good
safeties going forward in the context of Score V2 (guards against
selection in mod select overlays, against score submission with
the mod).
