Tweak backport of #32180 for 6.3

This commit fixes the build for 6.3 after the backport of #32180. First,
the meta plugin build no longer needed to call zip pom generation.
Second, pom generation for the jar needs to only happen when not
building a client jar, because gradle versions before 4.9 get confused
with multiple versions of the same artifact (see #32195).

Author: rjernst

buildSrc/src/main/groovy/org/elasticsearch/gradle/plugin/MetaPluginBuildPlugin.groovy

@@ -45,7 +45,6 @@ class MetaPluginBuildPlugin implements Plugin<Project> {
         }
         BuildPlugin.configurePomGeneration(project)
         project.afterEvaluate {
-            PluginBuildPlugin.addZipPomGeneration(project)
             if (isModule) {
                 if (project.integTestCluster.distribution == 'integ-test-zip') {
                     project.integTestCluster.module(project)

buildSrc/src/main/groovy/org/elasticsearch/gradle/plugin/PluginBuildPlugin.groovy

@@ -61,10 +61,11 @@ public class PluginBuildPlugin extends BuildPlugin {
                 // and generate a different pom for the zip
                 addClientJarPomGeneration(project)
                 addClientJarTask(project)
+            } else {
+                // while the jar isn't normally published, we still at least build a pom of deps
+                // in case it is published, for instance when other plugins extend this plugin
+                configureJarPom(project)
             }
-            // while the jar isn't normally published, we still at least build a pom of deps
-            // in case it is published, for instance when other plugins extend this plugin
-            configureJarPom(project)
 
             project.integTestCluster.dependsOn(project.bundlePlugin)
             project.tasks.run.dependsOn(project.bundlePlugin)