LUCENE-9490: skip checkMissingDocsDefault on runtime Java > 14.

gradle/testing/alternative-jdk-support.gradle

@@ -32,6 +32,9 @@ JavaInstallation altJvm = {
   }
 }()
 
+// Set up root project's property.
+rootProject.ext.runtimeJava = altJvm
+
 if (!currentJvm.javaExecutable.equals(altJvm.javaExecutable)) {
   // Set up java toolchain tasks to use the alternative Java.
   // This is a related Gradle issue for the future:
@@ -63,6 +66,7 @@ if (!currentJvm.javaExecutable.equals(altJvm.javaExecutable)) {
       options.forkOptions.javaHome = altJvm.installationDirectory.asFile
     }
 
+    // Javadoc compilation.
     def javadocExecutable = altJvm.jdk.get().javadocExecutable.asFile
     tasks.matching { it.name == "renderJavadoc" || it.name == "renderSiteJavadoc" }.all {
       dependsOn ":altJvmWarning"

gradle/validation/missing-docs-check.gradle

@@ -37,6 +37,18 @@ allprojects {
     task checkMissingDocsDefault(type: CheckMissingDocsTask, dependsOn: 'renderJavadoc') {
       dirs += [ project.javadoc.destinationDir ]
 
+      onlyIf {
+        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 ${maxSupported}.")
+          return false
+        } else {
+          return true
+        }
+      }
+
       // TODO: add missing docs for all classes and bump this to level=class
       if (project.path.startsWith(":solr")) {
         level = 'package'
@@ -72,7 +84,6 @@ configure(project(':lucene:core')) {
         "org/apache/lucene/index",
         "org/apache/lucene/codecs"
     ].collect { path -> file("${project.javadoc.destinationDir}/${path}") }
-
     checkMissingDocs {
       dependsOn checkMissingDocsMethod
     }