Make inner classes static when feasible

A static nested class does not keep an implicit reference to its
enclosing instance.

This prevents a common cause of memory leaks and uses less memory per
instance of the class.

Closes gh-28433

spring-web/src/main/java/org/springframework/http/server/reactive/UndertowHttpHandlerAdapter.java

@@ -88,7 +88,7 @@ public class UndertowHttpHandlerAdapter implements io.undertow.server.HttpHandle
 	}
 
 
-	private class HandlerResultSubscriber implements Subscriber<Void> {
+	private static class HandlerResultSubscriber implements Subscriber<Void> {
 
 		private final HttpServerExchange exchange;