use jdk21 virtual theads

### Search before asking

- [X] I searched in the [issues](https://github.com/apache/pulsar/issues) and found nothing similar.

### Motivation

I wanted to use virtual threads in the puslarcleint sdk to improve performance and reduce cpu consumption

### Solution

_No response_

### Alternatives

_No response_

### Anything else?

_No response_

### Are you willing to submit a PR?

- [ ] I'm willing to submit a PR!

Start by locating the Pulsar client SDK entry points that create or manage threads and check the project's supported JDK versions. The issue names no files, tests, benchmarks, or acceptance criteria, so clarify the intended scope and define how performance and CPU-consumption changes should be measured before starting.

Written by the indexing model from the issue text.