Remove JUnit 4 support

This commit also upgrades the test suite to use a more recent version as
removing support for JUnit4 effectively removes support for those as
well.

Closes gh-1219

initializr-actuator/src/test/java/io/spring/initializr/actuate/ActuatorIntegrationTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2021 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.
@@ -40,7 +40,7 @@ class ActuatorIntegrationTests extends AbstractFullStackInitializrIntegrationTes
 	void infoHasExternalProperties() {
 		String body = getRestTemplate().getForObject(createUrl("/actuator/info"), String.class);
 		assertThat(body).contains("\"spring-boot\"");
-		assertThat(body).contains("\"version\":\"2.1.4.RELEASE\"");
+		assertThat(body).contains("\"version\":\"2.4.4\"");
 	}
 
 }