[Java] Review code examples in documentation

[smintz]

This issue focuses on the validation of code examples in the Protoconf documentation for Java.

As we aim to deliver top-quality documentation to our users, we need to ensure all code examples provided are both correct and efficient. This requires thorough validation.

We need to validate the code in:

https://www.protoconf.dev/docs/0.1.7/consume-config-updates
https://www.protoconf.dev/docs/0.1.7/advanced-usage/mutation-service/grpc