Adapt ConcurrencyThrottleSupport to use ReentrantLock for better virtual thread support

[philwebb]

See spring-projects/spring-boot#39529, specifically:

The only possibility to limit the amount of parallel tasks is therefore the spring.task.execution.simple.concurrency-limit property, which essentially behaves like a max pool size. The issue I see here, is that ConcurrencyThrottleSupport, which is being used under the hood, uses the synchronized blocks which leads to thread pinning and might cause several issues with virtual threads. Maybe switching to ReentrantLock would mitigate that?