LUCENE-9490: Disable both tasks in :lucene:core on Java 15+

gradle/validation/missing-docs-check.gradle

@@ -15,6 +15,18 @@
  * limitations under the License.
  */
  
+def javaVersionCheck = {
+  def maxSupported = JavaVersion.VERSION_14
+  def runtimeVersion = runtimeJava.javaVersion
+  if (runtimeVersion > JavaVersion.VERSION_14) {
+    logger.warn("Skipping task because runtime Java version ${runtimeVersion} is " +
+        "higher than Java ${maxSupported}.")
+    return false
+  } else {
+    return true
+  }
+}
+
 allprojects {
   plugins.withType(JavaPlugin) {
     // Too many classes to fix overall to just enable the above to be level="method" right now,
@@ -37,17 +49,7 @@ allprojects {
     task checkMissingDocsDefault(type: CheckMissingDocsTask, dependsOn: 'renderJavadoc') {
       dirs += [ project.javadoc.destinationDir ]
 
-      onlyIf {
+      onlyIf javaVersionCheck
-        def maxSupported = JavaVersion.VERSION_14
-        def runtimeVersion = runtimeJava.javaVersion
-        if (runtimeVersion > JavaVersion.VERSION_14) {
-          logger.warn("Skipping task because runtime Java version ${runtimeVersion} is " +
-              "higher than Java ${maxSupported}.")
-          return false
-        } else {
-          return true
-        }
-      }
 
       // TODO: add missing docs for all classes and bump this to level=class
       if (project.path.startsWith(":solr")) {
@@ -72,6 +74,7 @@ configure(project(':lucene:core')) {
   // Defer until java plugin has been applied, otherwise we can't resolve project.javadoc.
   plugins.withType(JavaPlugin) {
     task checkMissingDocsMethod(type: CheckMissingDocsTask, dependsOn: 'renderJavadoc') {
+      onlyIf javaVersionCheck
       level = 'method'
     }