[Task] Build a version compatibility matrix for main repo and shenyu-client-java

### Description

## Background

After splitting Java client code into `shenyu-client-java`, users need to know which client versions work with which ShenYu main repo versions and which JDK versions are supported.

We need a documented compatibility matrix.

## Tasks

- [ ] Define client repo version dimensions.
- [ ] Define main repo/admin/bootstrap version dimensions.
- [ ] Define client-side JDK support policy.
- [ ] Define main repo JDK support policy.
- [ ] Identify supported, best-effort, and unsupported combinations.
- [ ] Document compatibility expectations for old client with new admin.
- [ ] Document compatibility expectations for new client with old admin.
- [ ] Add release checklist items for updating the matrix.

## Expected Output

A compatibility matrix document.

## Suggested Matrix Columns

- `shenyu-client-java` version
- ShenYu main repo version
- Client JDK baseline
- Client JDK verified versions
- Admin/bootstrap JDK baseline
- Registration protocol compatibility
- Notes

## Acceptance Criteria

- The matrix distinguishes runtime support from tested support.
- The matrix includes JDK 8, 17, 21, 23, and 25 where appropriate.
- The matrix can be updated during release.

### Task List

_No response_

Start by reviewing the shenyu-client-java and main repository release/version history, including the registration protocol and JDK support expectations. Produce the compatibility matrix covering the requested versions and support categories, then add release checklist guidance so the matrix can be updated with each release.

Written by the indexing model from the issue text.