Add default multiplier for mania key mods

[smoogipoo]

Resolves #30503

This doesn't go the full way. osu!stable will decrease the multiplier further if new_keys < old_keys, but this is hard to do here due to lack of ordering of calls to IApplicableToDifficulty.

• https://osu.ppy.sh/scores/3179253147

legacy: 859241
before: 959013
after: 863112

Those scoring values have been computed locally. Note that the "before" variant differs from the website because of a bug with not including mods correctly, that seems to have since been resolved but attributes haven't been reprocessed yet (#29772).

[bdach]

First of all:

!diffcalc
RULESET=mania
GENERATORS=score

Secondly: does this impact lazer scores at all? Because if it does, then a reverify is not enough. We'd need something like ppy/osu-queue-score-statistics#269.

[smoogipoo]

does this impact lazer scores at all?

Yes actually. That somehow totally slipped my mind.

[github-actions]

Target: #30506
Spreadsheet: https://docs.google.com/spreadsheets/d/1DdcdUi_BUcG3XSK92gNzMis92TyG7LaJBP7inGUnXto/edit