Rename baby zombie movement config option

This option does not set the absolute speed of the entity as the name
implies. It sets a modifier. The default (vanilla) value of `0.5` sets
the baby zombie to move at 50% faster than the base speed.

A negative value like `-0.4` would set them to move at 40% slower.

There should be no functional changes as a result of this change, it's
just clarifying the config name.

Spigot-Server-Patches/Undead-horse-leashing.patch

@@ -6,7 +6,7 @@ Subject: [PATCH] Undead horse leashing
 default false to match vanilla, but option to allow undead horse types to be leashed.
 
 diff --git a/src/main/java/com/destroystokyo/paper/PaperWorldConfig.java b/src/main/java/com/destroystokyo/paper/PaperWorldConfig.java
-index 30f0dcfd51..89556dc94a 100644
+index 970de3fe8..aaa1b60f9 100644
 --- a/src/main/java/com/destroystokyo/paper/PaperWorldConfig.java
 +++ b/src/main/java/com/destroystokyo/paper/PaperWorldConfig.java
 @@ -0,0 +0,0 @@ public class PaperWorldConfig {
@@ -20,7 +20,7 @@ index 30f0dcfd51..89556dc94a 100644
 +    }
  }
 diff --git a/src/main/java/net/minecraft/server/EntityHorseAbstract.java b/src/main/java/net/minecraft/server/EntityHorseAbstract.java
-index cb77d6bd21..b06bbc45cf 100644
+index 9c7e2437f..8aa60e95f 100644
 --- a/src/main/java/net/minecraft/server/EntityHorseAbstract.java
 +++ b/src/main/java/net/minecraft/server/EntityHorseAbstract.java
 @@ -0,0 +0,0 @@ public abstract class EntityHorseAbstract extends EntityAnimal implements IInven