Add Hidden as a relevant legacy mod for osu ruleset

[tsunyoku]

Required as a result of ppy/osu#31351.

Above change is a difficulty related change in the pp-dev branch, so this PR can be ignored until the next deploy cycle (where it will become a requirement).

[bdach]

This looks fine on its own, but the thing that I'm missing in all of this is that I don't see it mentioned anywhere that this seems to double-to-triple the required storage for beatmap attributes? Like, if Hidden and Spun Out now affect difficulty attributes, then as far as I can tell, this changes the number of combinations that will need to be stored from 54 to 144.

It's probably nowhere near mania levels, but my back-of-napkin math has this change increasing the storage size of osu! beatmap attributes from ~0.8GB to ~2.1GB. And probably increases the duration of recalculations in the future, too. Were these implications discussed at any point in review of the original change?

ping @peppy for awareness

[tsunyoku]

Yeah, that is true. In my head ppy/osu#31735 gave me a lot of headroom since we already made a lot of reductions to the space used since that PR.

If this sort of increase can't be justified, I can probably rollback the SpunOut part specifically (it's less important) if that brings the amount of combinations to a more acceptable number.

[smoogipoo]

Also if this change is to accepted, it should not go into master. We would need a similar pp-dev branch here.

I wonder if we should wait for #274 instead?

[tsunyoku]

As far as I was told, there's no plans to merge #274 soon unless there is specifically good reason for it. That's a separate discussion that I plan to touch on in that PR, but waiting on it might just end up being a blocker for the change that relies on this.

Also, I'm unsure what #274 will solve for this - HD and SO are both legacy mods so even with partial real time it'd continue to go to database. If you want to change that, then that's a huge consequence to the original scope of that PR.

Do we really need a pp-dev branch here for this? It's not really an often case where changes to osu-queue-score-statistics is required for PP, so leaving the PR to sit until it's required seems fine to me but I'm indifferent about it either way.

[tsunyoku]

As per ppy/osu#34838, this now only requires adding Hidden rather than Hidden and SpunOut.