KAFKA-7389: Enable spotBugs with Java 11 and disable false positive warnings (#5943)

See https://github.com/spotbugs/spotbugs/issues/756 for details on
the false positives affecting try with resources. An example is:

> RCN | Nullcheck of fc at line 629 of value previously dereferenced in
> org.apache.kafka.common.utils.Utils.readFileAsString(String, Charset)

Reviewers: Manikumar Reddy <manikumar.reddy@gmail.com>

build.gradle

@@ -149,7 +149,6 @@ subprojects {
   apply plugin: 'maven'
   apply plugin: 'signing'
   apply plugin: 'checkstyle'
-  if (!JavaVersion.current().isJava11Compatible())
   apply plugin: "com.github.spotbugs"
 
   sourceCompatibility = minJavaVersion
@@ -372,7 +371,6 @@ subprojects {
   }
   test.dependsOn('checkstyleMain', 'checkstyleTest')
 
-  if (!JavaVersion.current().isJava11Compatible()) {
   spotbugs {
     toolVersion = '3.1.8'
     excludeFilter = file("$rootDir/gradle/spotbugs-exclude.xml")
@@ -387,7 +385,6 @@ subprojects {
       html.enabled(!project.hasProperty('xmlSpotBugsReport') && !project.hasProperty('xmlFindBugsReport'))
     }
   }
-  }
 
   // Ignore core since its a scala project
   if (it.path != ':core') {

gradle/spotbugs-exclude.xml

@@ -23,6 +23,17 @@ This file dictates which categories of bugs and individual false positives that
 For a detailed description of spotbugs bug categories, see https://spotbugs.readthedocs.io/en/latest/bugDescriptions.html
 -->
 <FindBugsFilter>
+
+    <!-- false positive in Java 11, see https://github.com/spotbugs/spotbugs/issues/756 -->
+    <Match>
+        <Bug pattern="RCN_REDUNDANT_NULLCHECK_OF_NONNULL_VALUE"/>
+    </Match>
+
+    <!-- false positive in Java 11, see https://github.com/spotbugs/spotbugs/issues/756 -->
+    <Match>
+        <Bug pattern="RCN_REDUNDANT_NULLCHECK_WOULD_HAVE_BEEN_A_NPE"/>
+    </Match>
+
     <Match>
         <!-- Disable warnings about mutable objects and the use of public fields.
             EI_EXPOSE_REP: May expose internal representation by returning reference to mutable object