build: downgrade maven-sources-plugin from 3.3.0 to 3.2.1

The new version of the plugin caused build failure because of duplicate configurations.

Before fixing the root cause, downgrade to the old version.

pom.xml

@@ -164,6 +164,11 @@
           <artifactId>nexus-staging-maven-plugin</artifactId>
           <version>1.6.13</version>
         </plugin>
+        <plugin>
+          <groupId>org.apache.maven.plugins</groupId>
+          <artifactId>maven-source-plugin</artifactId>
+          <version>3.2.1</version>
+        </plugin>
       </plugins>
     </pluginManagement>