Change song select grouping to be divided into 10 BPM groups

[peppy]

As proposed in #34152.

[bdach]

Suggested change

	for (int i = 60; i < 300; i += 10)
	for (int i = 70; i < 300; i += 10)

Starting at i = 60 means there's an extra dead iteration of the loop every time.

[bdach]

This is sorta orthogonal to the issue here but I do wonder about using raw unrounded BPM, because on displays we use

osu/osu.Game/Utils/FormatUtils.cs

Lines 70 to 75 in 0d388cc

	/// <summary>
	/// Applies rounding to the given BPM value.
	/// </summary>
	/// <param name="baseBpm">The base BPM to round.</param>
	/// <param name="rate">Rate adjustment, if applicable.</param>
	public static int RoundBPM(double baseBpm, double rate = 1) => (int)Math.Round(baseBpm * rate);

and therefore stuff can be wonky (i.e. a 59.6 BPM beatmap being grouped into <60 BPM group but displaying as 60 BPM).

[peppy]

Hmm, seems like a good opportunity to fix / test coverage this.