Use ConcurrentHashMap.newKeySet

In places where a ConcurrentHashMap was used as a set by wrapping it
with Collections.newSetFromMap, switch to just using the set returned
by ConcurrentHashMap.newKeySet directly.

Closes gh-32294

spring-web/src/main/java/org/springframework/web/method/annotation/SessionAttributesHandler.java

@@ -64,7 +64,7 @@ public class SessionAttributesHandler {
 
 	private final Set<Class<?>> attributeTypes = new HashSet<>();
 
-	private final Set<String> knownAttributeNames = Collections.newSetFromMap(new ConcurrentHashMap<>(4));
+	private final Set<String> knownAttributeNames = ConcurrentHashMap.newKeySet(4);
 
 	private final SessionAttributeStore sessionAttributeStore;