Add classifier to version specific jar artifacts (#121083)

The mrjar plugin produces artifacts for each java version that may be
consumed downstream. This commit fixes an issue with gradle dependencies
when multiple java versions are depended on.

Author: rjernst

build-tools-internal/src/main/java/org/elasticsearch/gradle/internal/MrjarPlugin.java

@@ -163,6 +163,7 @@ private void addJar(Project project, SourceSet sourceSet, int javaVersion) {
         project.getConfigurations().register("java" + javaVersion);
         TaskProvider<Jar> jarTask = project.getTasks().register("java" + javaVersion + "Jar", Jar.class, task -> {
             task.from(sourceSet.getOutput());
+            task.getArchiveClassifier().set("java" + javaVersion);
         });
         project.getArtifacts().add("java" + javaVersion, jarTask);
     }