MINOR: Fix shadow jar publishing for the clients module (apache#15127)

The PR fixes the publishing of kafka-clients artifact to remote maven. The kafka-clients jar was recently shadowed which would publish the artifacts to the local maven repo successfully but would throw an error when publishing to remote maven. (as part of the release process)

The issue triggers only with publishMavenJavaPublicationToMavenRepository due to signing. Generating signed asc files error out for shadowed release artifacts as the module name (clients) differs from the artifact name (kafka-clients).

The fix is basically to explicitly define artifact of shadowJar to signing and publish plugin. project.shadow.component(mavenJava) previously outputs the name as client-<version>-all.jar though the classifier and archivesBaseName are already defined correctly in :clients and shadowJar construction.

build.gradle

@@ -303,7 +303,7 @@ subprojects {
             from components.java
           } else {
             apply plugin: 'com.github.johnrengelman.shadow'
-            project.shadow.component(mavenJava)
+            artifact shadowJar
           }
 
           afterEvaluate {