[MINOR] Drastically reducing concurrency level (to avoid CI flakiness) (apache#6754)

Author: Alexey Kudinkin

hudi-client/hudi-spark-client/src/test/java/org/apache/hudi/client/functional/TestHoodieBackedTableMetadata.java

@@ -112,7 +112,7 @@ public void testTableOperations(boolean reuseReaders) throws Exception {
   @ParameterizedTest
   @ValueSource(booleans = {true, false})
   public void testMultiReaderForHoodieBackedTableMetadata(boolean reuse) throws Exception {
-    final int taskNumber = 20;
+    final int taskNumber = 3;
     HoodieTableType tableType = HoodieTableType.COPY_ON_WRITE;
     init(tableType);
     testTable.doWriteOperation("000001", INSERT, emptyList(), asList("p1"), 1);