BeatmapBackground
Due to varying expectations in handling of `Beatmap`. Some places allow or expect null and some don't. Needs to be looked at closer separately.
IWorkingBeatmap.Background
GetBackground()
const
AimCount
Compatibility with osu-framework change https://github.com/ppy/osu-framework/pull/5512
SeasonalBackground
Closes #20065.
RemoveInternal
SeasonalBackgroundChanged