RFR: 8287749: Re-enable javadoc -serialwarn option

[Hannes Wallnöfer]

Please review a simple build change to re-enable the `-serialwarn` javadoc option for normal and reference API docs. The option was removed in [JDK-8252717](https://bugs.openjdk.org/browse/JDK-8252717) to avoid warnings for missing `@serial` tags which have since been added in [JDK-8286931](https://bugs.openjdk.org/browse/JDK-8286931). 

I tested normal and reference docs builds, both run without warning and the generated documentation is unchanged. The `@serial` tag in the Exception template is required and also does not affect javadoc output.

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Commit messages:
 - 8287749: Re-enable javadoc -serialwarn option

Changes: https://git.openjdk.org/jdk/pull/23802/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23802&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8287749
  Stats: 5 lines in 2 files changed: 2 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/23802.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23802/head:pull/23802

PR: https://git.openjdk.org/jdk/pull/23802