[FEATURE] should we support jdk21

### Code of Conduct

- [X] I agree to follow this project's [Code of Conduct](https://www.apache.org/foundation/policies/conduct)

### Search before asking

- [X] I have searched in the [issues](https://github.com/apache/incubator-uniffle/issues?q=is%3Aissue) and found no similar issues.

### Describe the feature

The main feature of JDK 21 is reducing thread overhead with virtual threads. We use several thread pools on the server side, like Netty, gRPC, and Flush. Is it necessary to carry out optimization diagnostics for JDK 21?

### Motivation

_No response_

### Describe the solution

_No response_

### Additional context

_No response_

### Are you willing to submit PR?

- [ ] Yes I am willing to submit a PR!

No files or tests are named. Start by locating the server-side Netty, gRPC, and Flush thread-pool entry points and assessing the JDK 21 virtual-thread question. Done requires an agreed support scope and concrete acceptance criteria for any resulting changes.

Written by the indexing model from the issue text.