Support Fabric version 1.21.3

[steviee]

Just added the new folder structure with some renamings.

Be aware, that Frabric-Api now uses a newer version of Loom which in turn needs a newer Version of Gradle.

Version changes

Loom 1.6.11 => Loom 1.8.11
Gradle 8.7 => Gradle 8.10.7

I feel there's some testing ahead of me...

[jacob1]

This doesn't compile for me.

.../dynmap/fabric-1.21.3/src/main/java/org/dynmap/fabric_1_21_3/FabricMapChunkCache.java:11: error: cannot find symbol
import net.minecraft.world.ChunkSerializer;
                          ^
  symbol:   class ChunkSerializer
  location: package net.minecraft.world
1 error

Seems like ChunkSerializer was removed during some big chunk loading refactors in 1.21.3. So the code that worked for fabric before will need to be refactored. (I don't use fabric, yet ..., just wanted to let you know it's broken)