4.x: Java target version clarification

We started out with Java 25 LTS level of support.

We bumped up to 26 once it released. However, I didn't see any enticing feature to be included on the API surface.

**We might fall back to 25 at the end. No code changes required.**

Depending on the progress and contents, we could also upgrade to 27 in September, but what it might feature is still to be discovered.