Adjust ForkJoinPool prefix in HdfsClientThreadLeakFilter

Changes "ForkJoinPool-" to "ForkJoinPool." in the
Thread getName().startsWith() checks in
HdfsClientThreadLeakFilter.  This resolves the
"There are still zombie threads that couldn't be terminated"
errors in the Hdfs IT tests.

Closes #127290
Closes #127289
Closes #127288
Closes #127287

Author: JeremyDahlgren

test/fixtures/hdfs-fixture/src/main/java/org/elasticsearch/test/fixtures/hdfs/HdfsClientThreadLeakFilter.java

@@ -35,6 +35,6 @@ public boolean reject(Thread t) {
             || t.getName().startsWith("SSL Certificates Store Monitor") // hadoop 3 brings that in
             || t.getName().startsWith("GcTimeMonitor") // hadoop 3
             || t.getName().startsWith("Command processor") // hadoop 3
-            || t.getName().startsWith("ForkJoinPool-"); // hadoop 3
+            || t.getName().startsWith("ForkJoinPool."); // hadoop 3
     }
 }