Add support for Virtual Threads

[fmbenhassine]

Virtual Threads (Project Loom) will be out of preview in JDK 21. This issue is to:

• Make sure that Spring Batch behaves in the same way with virtual threads as with platform threads
• Replace synchronized blocks/methods that contain blocking I/O operations with reentrant locks
• Check what should be updated in the Batch auto-configuration of Spring Boot with regard to virtual threads

[fmbenhassine]

• Test suite for virtual threads added in 0516808, tested successfully against JDK 21 EA Build 31 ✅
• Synchronized blocks/methods replaced with locks in 5bccfed ✅