build(java): add stable JPMS module name

[KirtiRamchandani]

Problem

flatbuffers-java currently lands on the module path as a filename-derived automatic module, so downstream module-info.java files see the unstable name flatbuffers.java and modular builds can warn about publishing with filename-based automodules.

Root cause

The Java artifact does not declare any JPMS module name metadata, while the Maven build still targets Java 8 (--release 8 on newer JDKs). Adding a top-level module-info.java would raise that source-level assumption.

Solution

Add Automatic-Module-Name: com.google.flatbuffers through the existing bundle plugin manifest instructions. This gives JPMS users a stable module name while preserving the Java 8 runtime/build target.

Addresses #9111.

Tests run

• mvn -f java/pom.xml -Dmaven.test.skip=true clean package
• inspected META-INF/MANIFEST.MF from java/target/flatbuffers-java-25.12.19.jar
• jar --describe-module --file java/target/flatbuffers-java-25.12.19.jar
• git diff --check