improve ThreadPerTaskExecutor clean up

[maxxedev]

In rare cases, ThreadPerTaskExecutor may cause memory leak. Improve clean up of threads

Make a change so that the code looks like this, before: https://github.com/thunkware/virtual-threads-backport/blob/f4b41439f45e7afa988c2b2aede2dfd71636a2f0/src/main/java/com/github/thunkware/ThreadProvider8.java#L78-L81

and after:

             Thread thread = threadFactory.newThread(() -> {
+                try {
+                    command.run();
+                } finally {
+                    threads.remove(Thread.currentThread());
+                }
             });