Migrate from traditional threads to virtual threads and update to JDK 21

[leonard1thecoder]

Task:

• Refactor the application to move from using traditional Java threads to virtual threads (Project Loom).
• Update the project to use JDK 21 for compatibility with virtual threads and latest Java features.

Benefits:

• Improved scalability and resource usage via lightweight concurrency model.
• Access to new features and performance enhancements in JDK 21.

Acceptance Criteria:

• All uses of traditional threads (e.g., new Thread(...), Executors.newFixedThreadPool(...)) are identified and migrated to use virtual threads (Thread.ofVirtual().start(...), Executors.newVirtualThreadPerTaskExecutor()).
• The codebase builds and passes tests on JDK 21.
• Documentation is updated to reflect the use of virtual threads and the JDK upgrade.

References:

• Virtual Threads documentation