Avoid pinning in SSEEmitter & ResponseBodyEmitter

#30996 fixed thread pinning issue. But only for SSEEmitter. Still there are several synchronized block in ResponseBodyEmitter.

Line 183 in e5b58ef

     public synchronized void send(Object object, @Nullable MediaType mediaType) throws IOException {  
 

Line 211 in e5b58ef

     public synchronized void send(Set<DataWithMediaType> items) throws IOException {  
 

To prevent thread pinning when using virtual thread, it have to replaced with ReentrantLock.

Many of us cannot move to JDK 24 yet (which reduces synchronized pinning), so we need a framework-level mitigation.

Provide feedback