[📑 Docs]: Rewrite “Creating a template – Java” tutorial to use AsyncAPI v3

### What Dev Docs changes are you proposing?

Update the “Creating a template – Java” tutorial to use AsyncAPI v3 instead of v2.
https://www.asyncapi.com/docs/tools/generator/generator-template-java

To align with the latest specification:

- The AsyncAPI document used in the tutorial needs to be updated to v3
- Generator template code must be updated to use the v3 parser API

A similar update was already done for the Python tutorial in #1826.

This ensures consistency with other language tutorials and prevents users from learning deprecated patterns.

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct

### Are you willing to work on this issue ?

Yes I am willing to submit a PR!

Start with the “Creating a template – Java” tutorial at the linked documentation page and compare its structure with the Python update in issue #1826. Update the tutorial's AsyncAPI document to v3 and revise the generator template code to use the v3 parser API; done means the Java tutorial no longer teaches v2 or deprecated parser patterns.

Written by the indexing model from the issue text.