[SPARK-30544][BUILD] Upgrade the version of Genjavadoc to 0.15

### What changes were proposed in this pull request?

Upgrade the version of Genjavadoc from 0.14 to 0.15.

### Why are the changes needed?

To enable to build for Scala 2.13.1.

### Does this PR introduce any user-facing change?

No.

### How was this patch tested?

I confirmed there is no dependency error related to genjavadoc by manual build.
Also, I generated javadoc by `LANG=C build/sbt -Pkinesis-asl -Pyarn -Pkubernetes -Phive-thriftserver  unidoc` for both code with/without this change and did `diff -r` target/javadoc.

Closes #27255 from sarutak/upgrade-genjavadoc.

Authored-by: Kousuke Saruta <sarutak@oss.nttdata.com>
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>

Author: sarutak

project/SparkBuild.scala

@@ -220,7 +220,7 @@ object SparkBuild extends PomBuild {
       .map(file),
     incOptions := incOptions.value.withNameHashing(true),
     publishMavenStyle := true,
-    unidocGenjavadocVersion := "0.14",
+    unidocGenjavadocVersion := "0.15",
 
     // Override SBT's default resolvers:
     resolvers := Seq(