Do not try to stop main thread during watchdog shutdown

In Java 21, Thread#stop is no longer implemented and wiill throw
an exception when called. As a result, we simply cannot halt
the main thread during shutdown anymore.

patches/server/Improved-Watchdog-Support.patch

@@ -108,15 +108,6 @@ index 0000000000000000000000000000000000000000..00000000000000000000000000000000
 +        // Paper start - kill main thread, and kill it hard
 +        shutdownThread = Thread.currentThread();
 +        org.spigotmc.WatchdogThread.doStop(); // Paper
-+        if (!isSameThread()) {
-+            MinecraftServer.LOGGER.info("Stopping main thread (Ignore any thread death message you see! - DO NOT REPORT THREAD DEATH TO PAPER)");
-+            while (this.getRunningThread().isAlive()) {
-+                this.getRunningThread().stop();
-+                try {
-+                    Thread.sleep(1);
-+                } catch (InterruptedException e) {}
-+            }
-+        }
 +        // Paper end
          // CraftBukkit end
          if (this.metricsRecorder.isRecording()) {