Fix generation of javadocs (elastic#76382)

The javadoc generation for painless api classes was broken by a recent
commit. This commit fixes it by correct the classpath used to run the
javadoc task. Additionally, the task is added to `check` to ensure it
runs with CI.

closes elastic#76373

modules/lang-painless/build.gradle

@@ -63,7 +63,7 @@ tasks.named("yamlRestCompatTest").configure {
  * Painless plugin */
 tasks.register("apiJavadoc", Javadoc) {
   source = sourceSets.main.allJava
-  classpath = sourceSets.main.runtimeClasspath + configurations.spi.classpath
+  classpath = sourceSets.main.compileClasspath + sourceSets.main.output
   include '**/org/elasticsearch/painless/api/'
   destinationDir = new File(docsDir, 'apiJavadoc')
 }
@@ -76,6 +76,9 @@ tasks.register("apiJavadocJar", Jar) {
 tasks.named("assemble").configure {
   dependsOn "apiJavadocJar"
 }
+tasks.named("check").configure {
+  dependsOn "apiJavadocJar"
+}
 
 tasks.named("bundlePlugin").configure {
   it.into("spi") {