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/mod-target.gradle

@@ -0,0 +1,15 @@
+import groovy.json.JsonSlurper
+
+File irisDirectory = project.name.startsWith('iris-') ? file('../..') : rootDir
+String targetMinecraft = providers.gradleProperty('minecraftVersion').getOrElse('26.3')
+Map<String, Map<String, Object>> targets = (Map<String, Map<String, Object>>) new JsonSlurper()
+    .parse(new File(irisDirectory, 'gradle/mod-targets.json'))
+Map<String, Object> target = targets.get(targetMinecraft)
+if (target == null) {
+    throw new GradleException("Unsupported Iris mod target ${targetMinecraft}; choose ${targets.keySet().join(', ')}")
+}
+ext.irisModTarget = target
+ext.irisModTargets = targets
+if (project.name.startsWith('iris-')) {
+    layout.buildDirectory.set(layout.projectDirectory.dir("build/${targetMinecraft}"))
+}