Minor: Set javadoc to legacymode

[Fokko]

This will avoid an error:

[INFO] Reactor Summary for Apache Avro Java 1.12.0-SNAPSHOT:
[INFO]
[INFO] Apache Avro Java ................................... SUCCESS [  2.043 s]
[INFO] Apache Avro ........................................ SUCCESS [  8.623 s]
[INFO] Apache Avro Android Compatibility .................. SUCCESS [  1.571 s]
[INFO] Apache Avro IDL .................................... SUCCESS [  5.358 s]
[INFO] Apache Avro Compiler ............................... SUCCESS [  3.918 s]
[INFO] Apache Avro Maven Plugin ........................... SUCCESS [  4.313 s]
[INFO] Apache Avro IPC .................................... SUCCESS [  5.228 s]
[INFO] Apache Avro IPC Jetty .............................. SUCCESS [  2.087 s]
[INFO] Apache Avro IPC Netty .............................. SUCCESS [  2.386 s]
[INFO] Trevni Java ........................................ SUCCESS [  0.132 s]
[INFO] Trevni Java Core ................................... SUCCESS [  0.345 s]
[INFO] Apache Avro Mapred API ............................. SUCCESS [  8.466 s]
[INFO] Trevni Java Avro ................................... FAILURE [  2.180 s]
[INFO] Trevni Specification ............................... SKIPPED
[INFO] Apache Avro Tools .................................. SKIPPED
[INFO] Apache Avro Protobuf Compatibility ................. SKIPPED
[INFO] Apache Avro Thrift Compatibility ................... SKIPPED
[INFO] Apache Avro Maven Archetypes ....................... SKIPPED
[INFO] Apache Avro Maven Service Archetype ................ SKIPPED
[INFO] Apache Avro gRPC ................................... SKIPPED
[INFO] Avro Integration Tests ............................. SKIPPED
[INFO] Apache Avro Codegen Test dependencies .............. SKIPPED
[INFO] Apache Avro Codegen Test ........................... SKIPPED
[INFO] Apache Avro Performance Test Suite ................. SKIPPED
[INFO] Apache Avro interop-data-test ...................... SKIPPED
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  47.480 s
[INFO] Finished at: 2024-07-24T10:18:12Z
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-javadoc-plugin:3.8.0:jar (module-javadocs) on project trevni-avro: Execution module-javadocs of goal org.apache.maven.plugins:maven-javadoc-plugin:3.8.0:jar failed: Unable to derive module descriptor for /home/fokko.driesprong/.m2/repository/org/apache/hadoop/thirdparty/hadoop-shaded-protobuf_3_7/1.1.1/hadoop-shaded-protobuf_3_7-1.1.1.jar: hadoop.shaded.protobuf.3.7: Invalid module name: '3' is not a Java identifier -> [Help 1]
[ERROR]
[ERROR] To see the full stack trace of the errors, re-run Maven with the -e switch.
[ERROR] Re-run Maven using the -X switch to enable full debug logging.
[ERROR]
[ERROR] For more information about the errors and possible solutions, please read the following articles:
[ERROR] [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/PluginExecutionException
[ERROR]
[ERROR] After correcting the problems, you can resume the build with the command
[ERROR]   mvn <args> -rf :trevni-avro

The rootcause of this issue seems to come from Hadoop.

What is the purpose of the change

(For example: This pull request improves file read performance by buffering data, fixing AVRO-XXXX.)

Verifying this change

(Please pick one of the following options)

This change is a trivial rework / code cleanup without any test coverage.

(or)

This change is already covered by existing tests, such as (please describe tests).

(or)

This change added tests and can be verified as follows:

(example:)

• Extended interop tests to verify consistent valid schema names between SDKs
• Added test that validates that Java throws an AvroRuntimeException on invalid binary data
• Manually verified the change by building the website and checking the new redirect

Documentation

• Does this pull request introduce a new feature? (yes / no)
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)