Fix premature player kicks on shutdown

When the server is stopping, the default execution handler method will throw a
RejectedExecutionException in order to prevent further execution, this causes
us to lose the actual kick reason. To mitigate this, we'll use a seperate marked
class in order to gracefully ignore these.

paper-server/src/main/java/io/papermc/paper/util/ServerStopRejectedExecutionException.java

@@ -0,0 +1,20 @@
+package io.papermc.paper.util;
+
+import java.util.concurrent.RejectedExecutionException;
+
+public class ServerStopRejectedExecutionException extends RejectedExecutionException {
+    public ServerStopRejectedExecutionException() {
+    }
+
+    public ServerStopRejectedExecutionException(final String message) {
+        super(message);
+    }
+
+    public ServerStopRejectedExecutionException(final String message, final Throwable cause) {
+        super(message, cause);
+    }
+
+    public ServerStopRejectedExecutionException(final Throwable cause) {
+        super(cause);
+    }
+}