Replace Jetty 9 only constant with explicit value

Closes gh-27121

Author: rstoyanchev

spring-websocket/src/main/java/org/springframework/web/socket/server/jetty/Jetty10RequestUpgradeStrategy.java

@@ -29,7 +29,6 @@
 
 import org.aopalliance.intercept.MethodInterceptor;
 import org.aopalliance.intercept.MethodInvocation;
-import org.eclipse.jetty.websocket.server.HandshakeRFC6455;
 
 import org.springframework.aop.framework.ProxyFactory;
 import org.springframework.aop.target.EmptyTargetSource;
@@ -56,7 +55,7 @@
  */
 public class Jetty10RequestUpgradeStrategy implements RequestUpgradeStrategy {
 
-	private static final String[] SUPPORTED_VERSIONS = new String[] { String.valueOf(HandshakeRFC6455.VERSION) };
+	private static final String[] SUPPORTED_VERSIONS = new String[] { String.valueOf(13) };
 
 	private static final Class<?> webSocketCreatorClass;
 

spring-websocket/src/main/java/org/springframework/web/socket/server/jetty/JettyRequestUpgradeStrategy.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2020 the original author or authors.
+ * Copyright 2002-2021 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -31,7 +31,6 @@
 
 import org.eclipse.jetty.websocket.api.WebSocketPolicy;
 import org.eclipse.jetty.websocket.api.extensions.ExtensionConfig;
-import org.eclipse.jetty.websocket.server.HandshakeRFC6455;
 import org.eclipse.jetty.websocket.server.WebSocketServerFactory;
 
 import org.springframework.context.Lifecycle;
@@ -168,7 +167,7 @@ public boolean isRunning() {
 
 	@Override
 	public String[] getSupportedVersions() {
-		return new String[] { String.valueOf(HandshakeRFC6455.VERSION) };
+		return new String[] { String.valueOf(13) };
 	}
 
 	@Override