Properly guard LogbackLoggingSystem

Previously, LoggingSystem#get would chose Logback by the sole presence
of a class in logback-core, with the assumption that logback-classic is
also on the classpath. An app that only had the former would therefore
fail.

This commit updates the condition to check for a class in
logback-classic instead.

Closes gh-26711

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/logging/LoggingSystem.java

 /*
- * 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.
@@ -60,7 +60,8 @@ public abstract class LoggingSystem {
 
 	static {
 		Map<String, String> systems = new LinkedHashMap<>();
-		systems.put("ch.qos.logback.core.Appender", "org.springframework.boot.logging.logback.LogbackLoggingSystem");
+		systems.put("ch.qos.logback.classic.LoggerContext",
+				"org.springframework.boot.logging.logback.LogbackLoggingSystem");
 		systems.put("org.apache.logging.log4j.core.impl.Log4jContextFactory",
 				"org.springframework.boot.logging.log4j2.Log4J2LoggingSystem");
 		systems.put("java.util.logging.LogManager", "org.springframework.boot.logging.java.JavaLoggingSystem");