Reduce ramp mod multipliers in line with other difficulty change mods for now

Closes https://github.com/ppy/osu/issues/20204.

Will require reprocessing of everything server-side.

osu.Game/Rulesets/Mods/ModTimeRamp.cs

@@ -19,6 +19,8 @@ namespace osu.Game.Rulesets.Mods
         /// </summary>
         public const double FINAL_RATE_PROGRESS = 0.75f;
 
+        public override double ScoreMultiplier => 0.5;
+
         [SettingSource("Initial rate", "The starting speed of the track")]
         public abstract BindableNumber<double> InitialRate { get; }