Remove support for Jetty 9.4.21 and earlier

Closes gh-18692

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/web/embedded/jetty/JettyServlet9419WebServerFactoryTests.java

@@ -1,43 +0,0 @@
-/*
- * Copyright 2012-2019 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.
- * You may obtain a copy of the License at
- *
- *      https://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
-
-package org.springframework.boot.web.embedded.jetty;
-
-import org.eclipse.jetty.server.handler.ErrorHandler;
-import org.junit.jupiter.api.Test;
-
-import org.springframework.boot.testsupport.classpath.ClassPathExclusions;
-import org.springframework.boot.testsupport.classpath.ClassPathOverrides;
-
-import static org.assertj.core.api.Assertions.assertThat;
-
-/**
- * Tests for {@link JettyServletWebServerFactory} with Jetty 9.4.19.
- *
- * @author Phillip Webb
- */
-@ClassPathExclusions({ "jetty-*.jar", "tomcat-embed*.jar" })
-@ClassPathOverrides({ "org.eclipse.jetty:jetty-io:9.4.19.v20190610", "org.eclipse.jetty:jetty-server:9.4.19.v20190610",
-		"org.eclipse.jetty:jetty-servlet:9.4.19.v20190610", "org.eclipse.jetty:jetty-util:9.4.19.v20190610",
-		"org.eclipse.jetty:jetty-webapp:9.4.19.v20190610", "org.mortbay.jasper:apache-jsp:8.5.40" })
-class JettyServlet9419WebServerFactoryTests extends AbstractJettyServletWebServerFactoryTests {
-
-	@Test
-	void correctVersionOfJettyUsed() {
-		assertThat(ErrorHandler.class.getPackage().getImplementationVersion()).isEqualTo("9.4.19.v20190610");
-	}
-
-}