Update spotbugs

.github/workflows/maven.yml

@@ -16,7 +16,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        jdk: ['8', '11', '17']
+        jdk: ['8', '11', '17', '21']
         os: [ubuntu-latest, windows-latest]
 
     steps:

fop-sandbox/src/tools/resources/findbugs/exclusions.xml

@@ -54,7 +54,7 @@
     <Bug pattern="DM_DEFAULT_ENCODING"/>
   </Match>
   <Match>
-    <Bug pattern="EI_EXPOSE_REP"/>
+    <Bug pattern="EI_EXPOSE_REP,EI_EXPOSE_REP2,MC_OVERRIDABLE_METHOD_CALL_IN_CONSTRUCTOR"/>
   </Match>
   <Match>
     <Bug pattern="SIC_INNER_SHOULD_BE_STATIC_ANON"/>

pom.xml

@@ -20,7 +20,7 @@
     <commons.logging.version>1.3.0</commons.logging.version>
     <compiler.plugin.version>3.11.0</compiler.plugin.version>
     <exec.plugin.version>1.4.0</exec.plugin.version>
-    <findbugs.plugin.version>4.2.3</findbugs.plugin.version>
+    <findbugs.plugin.version>4.7.3.6</findbugs.plugin.version>
     <jar.plugin.version>3.1.1</jar.plugin.version>
     <java.version>8</java.version>
     <junit.version>4.11</junit.version>