Refactor forge and example run configs based on MinecraftForge/ForgeGradle#546

- Updated run config formats based on the refactored setup
- Removed old 'runclient' and 'runserver' JavaExec tasks, replacing them by creating the run configs using the refactored setup.
- Set 'assetIndex' environment variable from FG
- Updated MDK for new run config setup

src/userdev/java/net/minecraftforge/userdev/UserdevLauncher.java

@@ -65,7 +65,7 @@ public class UserdevLauncher
             launchArgs = ObjectArrays.concat(launchArgs, new String[] {
                     "--accessToken", "blah",
                     "--version", "FMLDev",
-                    "--assetIndex", "1.13.1",
+                    "--assetIndex", System.getenv("assetIndex"),
                     "--assetsDir", assets,
                     "--userProperties", "{}"
             }, String.class);