Avoid pinning in SSEEmitter & ResponseBodyEmitter

[acktsap]

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

spring-framework/spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/method/annotation/ResponseBodyEmitter.java

Line 183 in e5b58ef

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

spring-framework/spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/method/annotation/ResponseBodyEmitter.java

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.

[bclozel]

Superseded by #35423