Add warning on Java language binding that it isn't maintained

### Describe the bug

The doc doesn't refer to the fact that Java library isn't maintained (https://github.com/fluent/fluent-logger-java last updated 4.5 years ago, issue https://github.com/fluent/fluent-logger-java/issues/99 left unanswered),

### Link to the problematic documentation

https://docs.fluentd.org/language-bindings/java

### Expected explanation

Add a note that the library isn't maintained (or up for adoption)

### Additional context

_No response_