[SPARK-36171][BUILD] Upgrade GenJavadoc to 0.18

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

This PR upgrades `GenJavadoc` plugin from `0.17` to `0.18`.

### Why are the changes needed?

`0.18` includes a bug fix for `Scala 2.13`.
```
This release fixes a bug (#286) with Scala 2.13.6 in relation with deprecated annotations in Scala sources leading to a NoSuchElementException in some cases.
```
https://github.com/lightbend/genjavadoc/releases/tag/v0.18

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

No.

### How was this patch tested?

Built the doc for Scala 2.13.
```
build/sbt -Phive -Phive-thriftserver -Pyarn -Pmesos -Pkubernetes -Phadoop-cloud -Pspark-ganglia-lgpl -Pkinesis-asl -Pdocker-integration-tests -Pkubernetes-integration-tests -Pscala-2.13 unidoc
```

Closes #33383 from sarutak/upgrade-genjavadoc-0.18.

Authored-by: Kousuke Saruta <sarutak@oss.nttdata.com>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>
(cherry picked from commit ad744fb4bf)
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>

project/SparkBuild.scala

@@ -265,7 +265,7 @@ object SparkBuild extends PomBuild {
       .orElse(sys.props.get("java.home").map { p => new File(p).getParentFile().getAbsolutePath() })
       .map(file),
     publishMavenStyle := true,
-    unidocGenjavadocVersion := "0.17",
+    unidocGenjavadocVersion := "0.18",
 
     // Override SBT's default resolvers:
     resolvers := Seq(