[Java] Provide a migration guide for the breaking changes introduced in 26.0

[blakeli0]

What version of protobuf and what language are you using?
Version: v3.25.3
Language: Java

Describe the problem you are trying to solve.

Protocol Buffers v26.0 introduced a lot of breaking changes for Java. To prepare for the upgrade, we are trying to replace the usage of all the removed methods before the upgrade. However, there is no official migration guide for v26.0, and it's usually not easy to find direct replacements for the removed methods either due to lack of Javadoc.
For example, OneofDescriptor.isSynthetic() was deprecated and now de-scoped, but there was no Javadoc or release notes hinting for a direct replacement. We tried our best to fix it in googleapis/sdk-platform-java#2764 by reading the source code, but we are not 100% confident and it would be great if there is something official for us to follow.

What we are asking for is an official guide that provides direct replacements or alternatives for all the removed methods. There is already an issue #16452 asking for a migrating path, but that issue is more for projects without a direct Protobuf dependency, focused on resolving runtime issues. This issue is more for projects with a direct Prorobuf dependency, and focused on compilation issues.

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