[SPARK-31547][BUILD] Upgrade Genjavadoc to 0.16

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

This PR aims to upgrade Genjavadoc to 0.16.

### Why are the changes needed?

Although we skipped Scala 2.12.11, this brings 2.12.11 official support and better 2.12.12 compatibility.

- https://github.com/lightbend/genjavadoc/commits/v0.16

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

No. (The generated doc is the same)

### How was this patch tested?

Build with 0.15 and 0.16.
```
$ SKIP_PYTHONDOC=1 SKIP_RDOC=1 SKIP_SQLDOC=1 jekyll build
```

Compare the result. The generated doc is identical.
```
$ diff -r _site_0.15 _site_0.16 | grep -v '^diff -r' | grep -v 'Generated by javadoc' | sort | uniq
---
5c5
```

Closes #28321 from dongjoon-hyun/SPARK-31547.

Authored-by: Dongjoon Hyun <dongjoon@apache.org>
Signed-off-by: Kousuke Saruta <sarutak@oss.nttdata.com>

project/SparkBuild.scala

@@ -221,7 +221,7 @@ object SparkBuild extends PomBuild {
       .map(file),
     incOptions := incOptions.value.withNameHashing(true),
     publishMavenStyle := true,
-    unidocGenjavadocVersion := "0.15",
+    unidocGenjavadocVersion := "0.16",
 
     // Override SBT's default resolvers:
     resolvers := Seq(