HADOOP-16866. Upgrade spotbugs to 4.0.6. (#2146)

hadoop-project/pom.xml

@@ -98,7 +98,7 @@
     <zookeeper.version>3.5.6</zookeeper.version>
     <curator.version>4.2.0</curator.version>
     <findbugs.version>3.0.5</findbugs.version>
-    <spotbugs.version>3.1.0-RC1</spotbugs.version>
+    <spotbugs.version>4.0.6</spotbugs.version>
     <dnsjava.version>2.1.7</dnsjava.version>
 
     <guava.version>27.0-jre</guava.version>