Add back `BaseDifficulty` with obsoletion counter to account for custom rulesets
Add back `BaseDifficulty` with obsoletion counter to account for custom rulesets

NeoAdonis/osu,ppy/osu,NeoAdonis/osu,NeoAdonis/osu,ppy/osu,ppy/osu,peppy/osu,peppy/osu,peppy/osu