[Task] Design the main repo JDK upgrade path for admin/bootstrap/runtime

### Description

## Background

The main ShenYu repository should be able to evolve admin, bootstrap, gateway runtime, and plugins independently from Java client JDK 8 compatibility constraints.

We need a JDK upgrade plan for the main repo, especially if future runtime work depends on JDK 21 features such as virtual threads.

## Tasks

- [ ] Inventory current JDK constraints in the main repository.
- [ ] Identify modules that can move to JDK 21+.
- [ ] Identify modules that must remain lower-JDK compatible, if any.
- [ ] Review Maven compiler/toolchain configuration.
- [ ] Review Docker image JDK versions.
- [ ] Review CI matrix and release workflow.
- [ ] Identify third-party dependencies affected by JDK upgrade.
- [ ] Identify examples and integration tests affected by JDK upgrade.
- [ ] Propose a staged migration plan.

## Expected Output

A JDK upgrade design document covering:

- Current JDK baseline
- Target JDK baseline
- Affected modules
- Build/CI changes
- Docker/runtime changes
- Dependency risks
- Rollback strategy
- Migration phases

## Acceptance Criteria

- The plan makes clear that client JDK 8 compatibility is handled in `shenyu-client-java`.
- The plan identifies risks before implementation starts.
- The plan includes at least one incremental migration phase.

### Task List

_No response_

Start by inventorying JDK constraints in the main repository, Maven compiler/toolchain configuration, Docker image versions, CI and release workflows, dependencies, examples, and integration tests. Treat shenyu-client-java as the JDK 8 compatibility boundary. Done means a design document covering baselines, affected modules, risks, rollback, and at least one incremental migration phase.

Written by the indexing model from the issue text.