Added 1.21.1 mod targets

- Build Fabric, Forge and NeoForge per target listed in gradle/mod-targets.json (1.21.1, 26.3)
- Add 1.21.1 adapter source overlays under adapters/*/src/versions/1.21.1
- Add verifyModdedMatrix and per-target Java runtime verification of loader jars
- Re-inject persistent dimensions from the levels-ready hook before the watchdog starts
- Wait for queued chunks and the final save before pregen completes
- Add the 1.21.1 data version and datapack supported_formats range
- Cascade regions whose sea or shore pool loses every biome
- Use Gson calls available on the 1.21.1 runtime

gradle.properties

@@ -25,6 +25,3 @@ irisVersion=4.3.3
 minecraftVersion=26.3
 # Bukkit plugin api-version: lowest supported Minecraft release line so one artifact loads on 26.1.2 through 26.3
 apiVersion=26.1
-fabricLoaderVersion=0.19.5
-forgeVersion=26.3-66.0.8
-neoForgeVersion=26.3.0.33-beta